---start physio 2.21.97---

renal physio continued.

yesterday we left off with the reabsorption of glucose. we defined a TM 
system...and today we're going to look at another tm system dealing 
w/secretion. 

so you can have TM for absorption or secretion. 
if you have excessive glucose in plasma so filtered load is higher than 
normal, and you have a TM system, so a limited # of solutes can be 
reabsorbed, there can be so much glucose not reabsorbed that it acts as 
osmotic force ot hold water in the lumen of nephron, and that water whch 
isn't reabsorbed will hold other solutes, and they are all lost into 
urine - eg electrolytes, etc.
this is typical of diabetes mellitus when bg reaches 300-400 mg/dl, and 
they undergo "osmotic diuresis". mannitol, like glucose at high 
concentrations, is an osmotic diuretic. these patients have tremendous 
urine volume, are PU/PD, and over several weeks lose considerable amts of 
electrolytes. this is reversible when plasma glucose level returns to 
normal. need insulin for that...insulin is true regulator of plasma 
glucose concentration.

reabsorption of amino acids: we're going to cover this quickly. it's a 
very tight system, can sort of be taken for granted. AAs are in plasma 
and kidney gets them at a relatively preset proportion. AAs are essential 
building blocks for body, and kidney tries to reabsorb 99.99% of them. 
there are redundant mechanisms for doing that. there is a transport 
system for AAs - based on type. a system for acid aas, basic aas, neutral 
aa; and there are backup systems that absorb several from each group, so 
that an animal won't lose all of a type if loses one mechanism. transport 
primarily at brush border, related to Na+ reabsorption. the active 
reabsorption is a modified tm system, so if you increase [aa] in plasma, 
you can overload one of these systems, causing spillover into the urine. 
the last line on handout under AA - competitive inhibition...we see there 
is a sharing of the transport systems. if you overload one system, you 
will turn down the reabsorption of other systems as well, by overloading 
those systems w/aa's usually absorbed by the overloaded system. not much 
clinical significance to this. some tubular dz in dogs.

organic anions, urate.
organic anions have a transport system that's always there, very 
efficient,handles many endogenous substances. most of these systems are 
bidirectional, similar to potassium. see list of endogenous compounds on 
handout . this system is a system that recognizes solutes by their 
electrical and chemical configs. system will take in a whole bunch of 
molecules that present the right interface. .AAs, benzoate, bile acids, 
cAMP, long chain fatty acids, hippurate, bydroxybenzoate, oxalate, 
prostoglandins, metabolic intermediates, etc. these are actively 
collected and secreted as a group and there is competition between them. 
you could overload a system w/long chain FAs by raising plasma 
concentration, and you'll find that then you've also disrupted transport 
of other solutes - they are usig shared resources - note that usually 
secretion overrides reabsorption.

also in table 13-1 is a list of drugs which are also secreted by this 
renal organic anion transport system.

acetazolamine, cephalothin, chlorothiazide, ethacrynic acid, 
fluoresceinate, furosemide, indomethacin, pen g, probenecid


 realize some drugs bind proteins and then are not filtered at 
glomerulus.. but many drugs have free and protein bound components. many 
drugs get into urine via transport system on basolateral membrane pulling 
drug out of peritubular capillary and secreting it into lumen. so we can 
concentrate this drug in the urine, which would be especially good to 
attack UTI bugs.

so this is one transport system which handles drugs- there is another 
system as well. all drugs are tested to see how they are 
excreted/metabolilzed, before being approved.

PAH - model solute for organic anion transport system - see fig 13.4.
PAH- is exogenous substance infused into animal for purpose of measureing 
RPF and as prototype for integrity/power of this system. if you infuse 
this into plasma, how much is free and how much goes into efferent art? 
well, hardly any is filtered. the part that's free in the blood goes 
through active transport related to dicarboxylate transport, related to 
sodium going into the cells, onthe basolateral side. the PAH- is actively 
taken into the cell, then passively goes across brush border into the 
urine. so we talk about a quantity of PAH- that is cleared by the kidney. 
the clearace of PAH- by chance represents the volume of renal plasma flow 
(RPF). 

C=UV/P (what?)

there is a classical
most of the PAH is secreted in proximal tubule
very little in distaltubule.

see fig 7-2 for graph of secretion
here they are challenging the system to see the power of the secretory 
system, measuring TM for PAH-
beginning w/plasma concentration on x axis and urine conc on y axis. 
w/plasma conc at 10 mg/dl, you see some is filtered, more is secreted, 
and more is excreted. when you reach a certain point n the plasma, a 
certain filtered load, you can't secrete any more....but you do excrete 
more. so there's a TM in mg/min/kg. this isn't used in clinical medicine 
but in pharmacology.

next substance that is commonly thought of as being handled by this 
system is urate. remember from biochem, see fig 7-1 purine pathways. 
these substances are the end products of purine metabolism - 
hypoxanthine, xanthine, and guanine, resulting in prod of uric acid or 
allantoin. these products, and intermediates, are all handled by this 
system and are typically secreted, not filterd. so kidney is integral in 
excretion of uricacid in most animals. in our patients there is also 
allantoin as an end product - primates do not make allantoin, we stop at 
uric acid, but other animals do make allantoin. an enzyme that breaks 
down uric acid: urikase - people don't have enough of that. dalmations 
are also deficient in urikase and they make mostly uric acid and have 
problems.

fig 7-4 shows urate prod in dalmations. nondalms on left, dalms on right. 
see that minimal uric acid appears in nondalms, but in dalms there is 
more uric acid excreted than was filtered, due to secretion and increased 
filtered load (plasma conc. higher)

other organic system: cation transport
ACH
choline
creatine
dopamine
norepi
epi 
histamine
(endogenous)

also drugs: atropine, cimetidine, morphine, neostigmine.

see fig 14-1 endogenous substances on secretory list shows AAs. but AAs 
are 99.99% absorbed! so this mechanism exists, but isn't usually turned 
on. but you can turn it on if needed.

that's about it - this will be covered in pharmacology.

moving on to urea.
this is not active transport.
in regards to renal concentrating capacity...

fig 13-10. nephron has a delivery of 100 solute particlesinto proximal 
tubule. where does that urea come from metabolically? end product of 
protein metabolism.made in liver, dumped into plasma. urea production is 
related to dietary protein intake. BUN increases with dietary protein 
intake. so here you have urea coming in at 100 particles. it's a small, 
very small molecule and is freelyfiltered. it's reabsorbed in proximal 
tubule passively. there is  NO active transport of urea in mammalian 
kidney. follows isosmotic reabsorption of water. at end of proximal 
tubule only about 30% of urea is left in lumen. then in loop of henle 
urea reenters the nephron. loop of henle ishighly permeable to urea. it's 
coming from collecting duct. so up in distal tubule you have 110 
particles...more than you started with. and about 40 end up in urine. 
about 40% of filtered load gets into urine, the rest is filtered by 
passive mechanisms. urea plays big role in interstitium of kidney 
-bolsters the hypertonicity of interstitium to allow concentration of 
urine. 

fig 12-11 in next handout...
long loop of henle, short loop of henle,and collecting duct, showing 
permeability to urea. huge perm in loop and collecting duct. 

fig 13-11 shows hgh urea conc in medulla pulls water out of collecting 
duct.

another thing about urea: BUN is also used clinically as gross indicator 
Of GFR, etc. normally about 25 mg/dl is BUN. animals w/renal failure with 
reduced GFR and RPF have dramatic elevations of BUN - 100-200  mg/dl due 
to inadequate filtration. will be discussed in medicine course.

next category:reabsorption of proteins by renal tubule.

used to be thought that small amt of protein which crosses glomerulus in 
health (in disesase lots more gets across) (up til 1985) we thought that 
that sm amt protein was destined to be lost in urine. there is a 
pinocytosis mechanism in brush border of prox and distal tubular cells 
which allows protein to adhere to brush border and be pulled into the 
cells. this uptake/digestion mechanism increases as  more protein gets 
into filtrate.

next is overview of solutes in filtrate:

see chart 13-12.
shows concentrations in different parts of nephron
all solutes begin at one and then they diverge. glucose, eg,bottoms out 
by the end of proximal tubule. [PAH] increases in prox tubule, and more 
in collecting duct. we know there's no active secretion of PAH in 
collecting duct, so how is PAH concentration increasing? Because water is 
LEAVING the collecting duct.

what is in the final urine?

this is not related to quantity that is filtered, this is just absolute 
conc. of solute in urine:

osmolarity  about 650, moderately concentrated. there will be cations, 
anions, and nonelectrolytes. 

cations and anions 180 and 155 mOsm, pretty close. LOTS of urea, 300 
mOsm, and 12 mOsm creatinine.

the cations/anions are the NH4+, K+, Na+, Organic acid, SO4--, H2PO4-, 
Cl-

so that's what is in there, but youhave to go through all the previously 
discussed mechansims to get there.

moving on to next handout:

urinary concentration is a fascinating subject because every animal on 
earth and in sea has capacity to do this. we all share the same water. 
the water in our cells and in air and urine has been circulating for 
aeons. it'sall the same water. urine osmolarity can go from 50-5000but 
usually more around 600-1000. ability to concentrate urine is related to 
ability to maintain ECFV. not all animals have same concentrating 
ability. fish don't have to concentrate well. desert animals really need 
to do it well. 

maximal urinary osmolarity chart:

beaver - about 500 mOsm maximum

this means when you take water away from the animal, this is as much as 
animal will ever concentrate the urine.

rat cat and dog concentrate better than man. 

man better than pig, beaver.

desert animals: kangaroo rat, jerboa, desert cat: can go up to 5000-6000 
mOsm. there's also a desert dog that can do that. this is due to length 
of loop of henle. rat loop of henle 2x as long as that of  man. kangaroo 
rat LOH 2x as long as regular rat.

factors controlling concentrating capacity: 9 major ones. 
fig 12-10 - left of p 23. permeability of nephron sites to movement of 
water.

y: permeability of nephrons to water
see descending LOH  very permeable, ascending hardly at all. collecting 
duct in between, but VERY permeable under influence of ADH.
fig 12-8: what solutes in interstitium allow urine to be concentrated? 
sodium, urea, and chloride.

if you look at urine and the inner and outer parts of the medulla, 
there's a continuing increase in concentrations of substance as you go 
into medulla. (during antidiuresis...water shortage). during diuresis ( 
water overload) the concentration of the solutes falls dramatically, 
primarily due to loss of urea into urine. when youvoluem expand, you turn 
off ADH, water stays in collecting duct, and urea goes with it. it's a 
wash out of the interstitium.

---break---

ok we're back 
on top of p 24 is urea permeability on nephron segments fg 12-11 which we 
saw earlier. 

god help me, if he continues in this vein i shall surely perish. (sorry. 
i'm feeling dramatic)

in inner zone of medulla, half of trapped solute is urea, and half is 
Na+Cl-

countercurrent exchange mechanism - nephron and vasa recta: explains how 
interstitium maintains concentration. 

vasa recta is lying on top of/next to limb of henle. vasa recta has 
peculiar permeability we don't fully understand. at cortical medullary 
junction, has same osmolarity as filtrate. then it gets high, maxing out 
at 1200 - ican't really tell what he's talking about because his pointer 
isn't working.

argh.

there must be a peculiar permeability orhormone mechanism in vasa recta 
which allows gradient to persist, so dont' try to fully understtand it, 
can't be done yet.

look at fig 9-5. it shows nephron. shows water taken out by vasa recta, 
not staying in interstitium.  note that vasa recta flow is counter 
current to flow in nephron and also is counter current to self, 
exchanging solute inside the loop. (?)

fig 9-4 - summary slide showing volume of water coming in (100) and going 
out (.5) and osmolarity throughout limb and in interstititum and sites of 
active transport of water.

important to maintain low medullary blood flow. if you don't,you get 
another wash out. blood going through quickly can remove excess solute. 
so two hormones ADH and prostoglandin E2.

ADH reduces medullary blood flow directly by acting as vasoconstrictor 
there.
prostoglandin E2 and GF2a are locally synthesized by interstitial and 
tubular cells, and antagonize ADH by acting as vasodilators. the ADH is 
an endocrine hormone, prostoglandines are autocrine - act locally, 
quickly regulated.

ADH (vasopressin) small peptide made in posterior pituitary, has variety 
of fx. 
post pituitary makes more of it when plasma osmolality increases (eg when 
animal becomes dehydrated). a one or two increase in [Na+] causes ADH 
release. if you measure ADH you will see it rises within minutes of this 
increase. if you look at jugular circulation, you'll see concentration is 
much higher (not yet diluted by systemic circulation) 

so major release mechanism is plasma osmolality. also volume expansion - 
volume receptors will turn down ADH productions.

site of action of vasopressin (ADH)- 5 sites. 
-glomerulus: ADH will cause mesangial contraction
- vasa recta - medullary blood flow vasoconstrictor
-electrolyte transport in ascending limb
(growth factor in embryonic life, btw)
- medullary interstitium - these cells produce prostoglandins which 
oppose the vasoconstriction
- permeability of collecting duct to water is increased.

cellular action of ADH
"docks" at basolateral side, causes kinase cascade in cell, opens water 
channels, allows water to move across cell from tubular lumen. water 
movement is actually kind of complex. involves fig 11-13 showing 
vesicular traffic - water droplets come in in these liquid membranes, are 
transported across and released, in an energy driven system regulated by 
adenyl cyclase, using special anatomic structures. water moves in 
"aggrephores" - massive movement of water.

fig 28-1: mechanism for forming dilute urine. one way; drink 5x normal 
amt water andyou'll make dilute urine. or, stop making ADH - like people 
with diabetes insipidus. those patients make dilute urine all the time so 
drink huge amts of water. note that you can also get water psychosis from 
drinking too much water.

what happens when you make dilute urine? you lose a lot of water into the 
urine, so you have lost ability to move urea out of collecting duct, so 
you lose 50-60% of urea, so you dissipate the interstitium - lessen the 
hypertonicity. you have a 700 mOsm interstitium but a 65 mOsm urine. this 
is easily reversed with drugs or reducing water intake. this is a 
temporarychange.

other factors:

central osmoreceptors and left atrial volume receptors and carotid sinus 
and aortic baroreceps and increased temp and stuff increase ADH synthesis 
and secretion.see fig 5

main hormone things in kidney:
renin angiotensin mechanism see fig 10-3 and list of factors controlling 
renin secretion (increased symp tone, etc.)

look at fig H1 on far left- shows makeup of the peptides (renin, ATI, 
II,III)
ATIII is not active, is broken down by angiotensinases)

see table one: list of ATII sites of action.

this sucks, i'm sorry. he's not TELLING us anythign. he's just telling us 
"look at the handout to figure out how this works."

see fig 14-11 schema showing ATII effects. the point of renin-at system 
is to ehlp maintain ECFV.

tissue renin-at system: nvolves exact same hormones, but not an endocrine 
system, an autocrine or paracrine system. first found in brain, then 
kidney, 10 yrs ago. many experiments assumed all renin originated at jg 
apparatus, then they found out other organs make some too..a redundant, 
local system. so in kidney, while JGA is involved, theres' also local 
production influencing blood flow and sodium reabsorption without ever 
entering the systemic circulation. blood vessels throughout the body also 
have this, so does brain, heart, ovary, and other organs. these systems 
are independent of endocrine system. just know we don't have to go 
through all systemic control mechanisms to explain the local effects. can 
respond locally in opposite direction from central endocrine system, or 
can enhance.

ENDOTHELIN
most potent vasoconstrictor in body. there are multiple endothelins. 
1,2,3. made by endothelial cells all over body, more at some sites, most 
importantly in preglomerular vessels - all of intrarenal vasculature from 
bifurcation of renal aa to glomerulus. not sure of pharmacology...being 
studied now. major antagonist is NO aka EDRF, also made in endothelial 
cells. note effects on BP, RPF< GFR. dose related response. increases 
MAP, decreases RPF and GFR (vasoconstriction).
see fig 1 - variety of factors acting on smooth muscle to change tone of 
vessel. both endocrine and autocrine. cyclooxygenase and precursors of 
prostoglandins, PGH2 and thromoxane, endothelin, EDCF, etc. just see how 
endothelin is in a position to enhance or compete w/vasoconstrictors 
present in all vascular smooth muscle. vasodilators not seen in this 
chart. major opposing factor is NO aka Endothelin Derived Relaxing 
Factor. NOis made locally as soon as local autocrine system makes 
endothelin, it will make NO.

biologic actions of endothelin in table 1a to the right. we've discussed 
some. read and ponder on your own.

another group of peptides: natriuretic peptides.
3 major forms. chemically similar, not identical

ANP-H (atriopeptin) comes from heart, right atrium. 
BNP comes from brain, lateral ventricles
Urodilantin- from renal tubular cells. an ANP like substance.

all NP molecules are 32 AA peptides. 
physiologic significance: have minor role in normal homeostasis, are made 
in excess during volume expansion, CHF, CRF, hypertension, etc.
will go to kidney and turn down Na+ reabsorption, causing natriuresis, 
diureses (sodium, water loss) and provide escape from mineralocorticoids. 
NPs jam the mineralocorticoid receptors. also inhibit neural stimulation 
of renin-at system. inhibit renin-at release in kidney and in brain.

vitamin d metabolism: kidney makes final form of D3. if kidney doesn't 
make enough D3 for whatever reason - it can't do it, or it isn't getting 
enough precursor - then animal can't absorb calcium and will get 
hypocalcemic over time. 

prostoglandins: made throughout kidney - very high conc in medulla. many 
cells in kidney including tubular cells willmake prostoglandins. also 
mesangial, endothelial, etc. PGE2 and PGF2 are most potent in kidney. act 
as vasodilators, antagonize ADH, antagonize renin-at system often.  they 
are relased by high presence of AII, reactive oxygen species, hypoxia, 
etc. balance between renin and prostoglandins seen in fig B. look and 
ponder and note that when you interfere w/prostoglandin system you see 
that you get more vasoconstriction due to AII overriding it. so you can 
see GI ulceration and kidney probs.

kallikrein-kinin system: another local hormonal system. 
production of this occurs in many organs and vascular beds, but all are 
independent, there is no endocrine production, all are destroyed locally. 
this is a complex system that is not totally understood. generally 
vasodilatory and antagonistic to ATII system. cohort of prostoglandin 
system to oppose ATII. fig 10-8 shows system components: renal tissue 
pre-kallikrein in tubular cells; activator splits into active kallikrein, 
converted to kinogen, broken down w/peptidase into bradykinin which is 
active vasodilator. bradykinin can be destroyed two ways. kininase II and 
kininase I. kininase II is ace (angiotensin converting enzyme). so the 
hormone that promotes ATII breaks down the antagonistic bradykinin. neat, 
huh?

several tubular effects of kinins. do have minimal role in reducing Na+ 
reabsorption, blunting effeccts of ADH, etc. see fig C. we aren't sure of 
howmuch of this is really significant. just get big picture.

renal prod of erythropoeitin - not being covered really. kidney regulates 
RBC production. in kidney failure, patients become anemic.

renal regulation of peptide hormones. know that all peptide hormones in 
circulation come to kidney in plasma and are presented to tubular cells, 
which decide how much to put in urine and how much to recycle.


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