ATP as a mediator of macula densa cell signalling

Within each nephro-vascular unit, the tubule returns to the vicinity of its own glomerulus. At this site, there are specialised tubular cells, the macula densa cells, which sense changes in tubular fluid composition and transmit information to the glomerular arterioles resulting in alterations in glomerular filtration rate and blood flow. Work over the last few years has characterised the mechanisms that lead to the detection of changes in luminal sodium chloride and osmolality by the macula densa cells. These cells are true “sensor cells” since intracellular ion concentrations and membrane potential reflect the level of luminal sodium chloride concentration. An unresolved question has been the nature of the signalling molecule(s) released by the macula densa cells. Currently, there is evidence that macula densa cells produce nitric oxide via neuronal nitric oxide synthase (nNOS) and prostaglandin E₂ (PGE₂) through cyclooxygenase 2 (COX 2)-microsomal prostaglandin E synthase (mPGES). However, both of these signalling molecules play a role in modulating or regulating the macula-tubuloglomerular feedback system. Direct macula densa signalling appears to involve the release of ATP across the basolateral membrane through a maxi-anion channel in response to an increase in luminal sodium chloride concentration. ATP that is released by macula densa cells may directly activate P2 receptors on adjacent mesangial cells and afferent arteriolar smooth muscle cells, or the ATP may be converted to adenosine. However, the critical step in signalling would appear to be the regulated release of ATP across the basolateral membrane of macula densa cells.     

Acute saline expansion increases nephron filtration and distal flow rate but maintains tubuloglomerular feedback responsiveness: role of adenosine A(1) receptors

Temporal adaptation of tubuloglomerular feedback (TGF) permits readjustment of the relationship of nephron filtration rate [single nephron glomerular filtration rate (SNGFR)] and early distal tubular flow rate (V(ED)) while maintaining TGF responsiveness. We used closed-loop assessment of TGF in hydropenia and after acute saline volume expansion (SE; 10% body wt over 1 h) to determine whether 1) temporal adaptation of TGF occurs, 2) adenosine A(1) receptors (A(1)R) mediate TGF responsiveness, and 3) inhibition of TGF affects SNGFR, V(ED), or urinary excretion under these conditions. SNGFR was evaluated in Fromter-Wistar rats by micropuncture in 1) early distal tubules (ambient flow at macula densa), 2) recollected from early distal tubules while 12 nl/min isotonic fluid was added to late proximal tubule (increased flow to macula densa), and 3) from proximal tubules of same nephrons (zero flow to macula densa). SE increased both ambient SNGFR and V(ED) compared with hydropenia, whereas TGF responsiveness (proximal-distal difference in SNGFR, distal SNGFR response to adding fluid to proximal tubule) was maintained, demonstrating TGF adaptation. A(1)R blockade completely inhibited TGF responsiveness during SE and made V(ED) more susceptible to perturbation in proximal tubular flow, but did not alter ambient SNGFR or V(ED). Greater urinary excretion of fluid and Na(+) with A(1)R blockade may reflect additional effects on the distal nephron in hydropenia and SE. In conclusion, A(1)R-independent mechanisms adjust SNGFR and V(ED) to higher values after SE, which facilitates fluid and Na(+) excretion. Concurrently, TGF adapts and stabilizes early distal delivery at the new setpoint in an A(1)R-dependent mechanism.     

Effects of atrial natriuretic factor on blood pressure and the renin-angiotensin-aldosterone system

Atrial natriuretic factor (ANF) antagonizes vasoconstriction induced by numerous smooth muscle agonists and also lowers blood pressure in intact animals. ANF has particularly marked relaxant effects on angiotensin II-contracted vessels in vitro. Sensitivity to the blood pressure-lowering effect of ANF in vivo appears to be enhanced in renin-dependent models of renovascular hypertension compared with other experimental hypertensive models. The depressor action of low, possibly physiological doses of ANF in two-kidney, one-clip Goldblatt rats is due to a decrease in total peripheral resistance. On the other hand, high doses of ANF can lower cardiac output, particularly in volume-expanded models such as deoxycorticosterone-salt hypertension. ANF markedly inhibits renin secretion in intact animals, probably via increased glomerular filtration rate and load of sodium chloride to the macula densa. This effect is masked when renal perfusion is impaired (e.g., via unilateral renal artery constriction), in which case ANF may stimulate renin secretion slightly. ANF also reduces plasma aldosterone in vivo and inhibits basal and agonist-induced aldosterone release from isolated adrenal cortical cells. This effect appears to be especially marked for angiotensin-induced aldosterone production in vivo and in vitro. These findings indicate that ANF has potentially important interactions with the renin-angiotensin-aldosterone system and suggest a role for ANF in the homeostatic control of blood pressure as well as of extracellular fluid volume.     

Nonlinear interactions in renal blood flow regulation

We have developed a model of tubuloglomerular feedback (TGF) and the myogenic mechanism in afferent arterioles to understand how the two mechanisms are coupled. This paper presents the model. The tubular model predicts pressure, flow, and NaCl concentration as functions of time and tubular length in a compliant tubule that reabsorbs NaCl and water; boundary conditions are glomerular filtration rate (GFR), a nonlinear outflow resistance, and initial NaCl concentration. The glomerular model calculates GFR from a change in protein concentration using estimates of capillary hydrostatic pressure, tubular hydrostatic pressure, and plasma flow rate. The arteriolar model predicts fraction of open K channels, intracellular Ca concentration (Ca(i)), potential difference, rate of actin-myosin cross bridge formation, force of contraction, and length of elastic elements, and was solved for two arteriolar segments, identical except for the strength of TGF input, with a third, fixed resistance segment representing prearteriolar vessels. The two arteriolar segments are electrically coupled. The arteriolar, glomerular, and tubular models are linked; TGF modulates arteriolar circumference, which determines vascular resistance and glomerular capillary pressure. The model couples TGF input to voltage-gated Ca channels. It predicts autoregulation of GFR and renal blood flow, matches experimental measures of tubular pressure and macula densa NaCl concentration, and predicts TGF-induced oscillations and a faster smaller vasomotor oscillation. There are nonlinear interactions between TGF and the myogenic mechanism, which include the modulation of the frequency and amplitude of the myogenic oscillation by TGF. The prediction of modulation is confirmed in a companion study (28).     

Renal blood flow, early distal sodium, and plasma renin concentrations during osmotic diuresis

Inconsistencies in previous reports regarding changes in early distal NaCl concentration (ED(NaCl)) and renin secretion during osmotic diuresis motivated our reinvestigation. After intravenous infusion of 10% mannitol, ED(NaCl) fell from 42.6 to 34.2 mM. Proximal tubular pressure increased by 12.6 mmHg. Urine flow increased 10-fold, and sodium excretion increased by 177%. Plasma renin concentration (PRC) increased by 58%. Renal blood flow and glomerular filtration rate decreased, however end-proximal flow remained unchanged. After a similar volume of hypotonic glucose (152 mM), ED(NaCl) increased by 3.6 mM, (P < 0.01) without changes in renal hemodynamics, urine flow, sodium excretion rate, or PRC. Infusion of 300 micromol NaCl in a smaller volume caused ED(NaCl) to increase by 6.4 mM without significant changes in PRC. Urine flow and sodium excretion increased significantly. There was a significant inverse relationship between superficial nephron ED(NaCl) and PRC. We conclude that ED(Na) decreases during osmotic diuresis, suggesting that the increase in PRC was mediated by the macula densa. The results suggest that the natriuresis during osmotic diuresis is a result of impaired sodium reabsorption in distal tubules and collecting ducts.     

The relationship between sodium excretion and renin secretion by the perfused kidney.

The effect of altered tubular sodium reabsorption on renin secretion (RSR) was examined under conditions in which other factors influencing renin release could be controlled or excluded. To do this, isolated canine kidneys were perfused at constant pressure with blood circulating from donor animals. Volume expansion or hemorrhage of the donor dogs produced large changes in the animal's blood pressure, renal function, sodium excretion (UNaV), and RSR, but were without effect on renal hemodynamics, UNaV, or RSR in the perfused kidney. Hemodilution without volume expansion, resulted in hypotension, decreased UNaV and increased RSR in the donor dogs, and increased UNaV and suppressed RSR in the perfused kidney. These effects of hemodilution in the perfused kidney were partially reversed when plasma protein concentration was restored to control levels with hyperoncotic albumin, and, overall, there was a significant inverse relationship between electrolyte excretion and RSR. These results provide new evidence for the hypothesis that the rate at which sodium is delivered to the macula densa is an important determinant of the rate of renin secretion.     

A major role for the EP4 receptor in regulation of renin

Prostaglandins have been implicated as paracrine regulators of renin secretion, but the specific pathways and receptor(s) carrying out these functions have not been fully elucidated. To examine the contributions of prostanoid synthetic pathways and receptors to regulation of renin in the intact animal, we used a panel of mice with targeted disruption of several key genes: cyclooxygenase-2 (COX-2), microsomal PGE synthases 1 and 2 (mPGES1, mPGES2), EP2 and EP4 receptors for PGE(2), and the IP receptor for PGI(2). To activate the macula densa signal for renin stimulation, mice were treated with furosemide over 5 days and renin mRNA levels were determined by real-time RT-PCR. At baseline, there were no differences in renin mRNA levels between wild-type and the various strains of mutant mice. Furosemide caused marked stimulation of renin mRNA expression across all groups of wild-type control mice. This response was completely abrogated in the absence of COX-2, but was unaffected in mice lacking mPGES1 or mPGES2. The absence of G(s)/cAMP-linked EP2 receptors had no effect on stimulation of renin by furosemide and there was only a modest, insignificant reduction in renin responses in mice lacking the IP receptor. By contrast, renin stimulation in EP4(-/-) mice was significantly reduced by ～70% compared with wild-type controls. These data suggest that stimulation of renin by the macula densa mechanism is mediated by PGE(2) through a pathway requiring COX-2 and the EP4 receptor, but not EP2 or IP receptors. Surprisingly, mPGES1 or mPGES2 are not required, suggesting other alternative mechanisms for generating PGE(2) in response to macula densa stimulation.     

Basic properties and potential regulators of the apical K+ channel in macula densa cells

These studies examine the properties of an apical potassium (K+) channel in macula densa cells, a specialized group of cells involved in tubuloglomerular feedback signal transmission. To this end, individual glomeruli with thick ascending limbs (TAL) and macula densa cells were dissected from rabbit kidney and the TAL covering macula densa cells was removed. Using patch clamp techniques, we found a high density (up to 54 channels per patch) of K+ channels in the apical membrane of macula densa cells. An inward conductance of 41.1 +/- 4.8 pS was obtained in cell-attached patches (patch pipette, 140 mM K+). In inside- out patches (patch pipette, 140 mM; bath, 5 mM K+), inward currents of 1.1 +/- 0.1 pA (n = 11) were observed at 0 mV and single channel current reversed at a pipette potential of -84 mV giving a permeability ratio (PK/PNa) of over 100. In cell-attached patches, mean channel open probability (N,Po, where N is number of channels in the patch and Po is single channel open probability) was unaffected by bumetanide, but was reduced from 11.3 +/- 2.7 to 1.6 +/- 1.3 (n = 5, p < 0.02) by removal of bath sodium (Na+). Simultaneous removal of bath Na+ and calcium (Ca2+) prevented the Na(+)-induced decrease in N.Po indicating that the effect of Na+ removal on N.Po was probably mediated by stimulation of Ca2+ entry. This interpretation was supported by studies where ionomycin, which directly increases intracellular Ca2+, produced a fall in N.Po from 17.8 +/- 4.0 to 5.9 +/- 4.1 (n = 7, p < 0.02). In inside- out patches, the apical K+ channel was not sensitive to ATP but was directly blocked by 2 mM Ca2+ and by lowering bath pH from 7.4 to 6.8. These studies constitute the first single channel observations on macula densa cells and establish some of the characteristics and regulators of this apical K+ channel. This channel is likely to be involved in macula densa transepithelial Cl- transport and perhaps in the tubuloglomerular feedback signaling process.     

Immunohistochemical localization of renin, NO synthase-1, and cyclooxygenase-2 in rodent kidney

The renin-angiotensin system (RAS) and tubuloglomerular feedback (TGF) are central to the maintenance of blood pressure and body fluid composition. Renin, NO synthase-1 (NOS-1), and cyclooxygenase-2 (COX-2) are key regulators of the RAS and TGF. In the present study, to investigate species-specific differences in the RAS and TGF, we immunohistochemically and morphometrically investigated the localization of renin, NOS-1, and COX-2 in the kidneys of various laboratory rodents and comparing males with females (DBA/2Cr mice, F344/N rats, Syrian hamsters, MON/JmsGbs gerbils and Hartley guinea pigs). In all animals, renin-positive immunoreactions were observed in the vascular walls of afferent arterioles. Renin immunoreactions appeared to be more widely distributed in mice. Mice had a greater number of renin-positive arterioles than other species. NOS-1-positive reactions were detected in the macula densa (MD) of all animals. Mice had the greatest number of NOS-1-positive MD cells. In addition to NOS-1-positive reactions, COX-2-positive reactions were observed in the MD of mice, rats, hamsters and gerbils. Interestingly, guinea pigs had no COX-2-positive MD cells. Rats had the greatest number of COX-2-positive MD cells. In nephron segments excluding the MD, the immunohistochemical localization of NOS-1 and COX-2 differed markedly among not only species but also sexes within the same species. In conclusion, we determined that localization of renin, NOS-1, and COX-2 showed large species- and sex-related differences. These data suggest that the regulation mechanisms of the RAS and TGF via renin, NOS-1, and COX-2 differ among rodents.     

Fluid waves in renal tubules.

Autoregulation of renal blood flow is ineffective when arterial pressure perturbations occur at frequencies above 0.05 Hz. To determine whether wave propagation velocity to the macula densa is rate limiting, we estimated compliances of the proximal tubule and the loop of Henle, and used these values in a model of pressure and flow as functions of time and distance in the nephron. Compliances were estimated from measurements of pressures and flows in early proximal, late proximal, and early distal tubules in rats under normal and Ringer-loaded conditions. A model of steady pressure and flow in a compliant, reabsorbing tubule was fitted to these results. The transient model was a set of nonlinear, hyperbolic partial differential equations with split, nonlinear boundary conditions, and was solved with finite difference methods. The loop of Henle compliance was larger than the proximal tubule compliance, and impulses in glomerular filtration rate were attenuated in magnitude and delayed in time in the loop of Henle. Simulated step forcings revealed a similar pattern. Periodic variations of GFR were attenuated at frequencies greater than 0.05 Hz, and there was a delay of 5 s between variations in GFR and macula densa flow rate. The high compliance of the loop slows wave propagation to the macular densa and reduces the amplitude of high frequency waves originating in the glomerulus, but other parts of the signal chain also contribute to the slow response of macula densa feedback.     

Abolished tubuloglomerular feedback and increased plasma renin in adenosine A1 receptor-deficient mice

The hypothesis that adenosine acting on adenosine A1 receptors (A1R) regulates several renal functions and mediates tubuloglomerular feedback (TGF) was examined using A1R knockout mice. We anesthetized knockout, wild-type, and heterozygous mice and measured glomerular filtration rate, TGF response using the stop-flow pressure (P(sf)) technique, and plasma renin concentration. The A1R knockout mice had an increased blood pressure compared with wild-type and heterozygote mice. Glomerular filtration rate was similar in all genotypes. Proximal tubular P(sf) was decreased from 36.7 +/- 1.2 to 25.3 +/- 1.6 mmHg in the A1R+/+ mice and from 38.1 +/- 1.0 to 27.4 +/- 1.1 mmHg in A1R+/- mice in response to an increase in tubular flow rate from 0 to 35 nl/min. This response was abolished in the homozygous A1R-/- mice (from 39.1 +/- 4.1 to 39.2 +/- 4.5 mmHg). Plasma renin activity was significantly greater in the A1R knockout mice [74.2 +/- 14.3 milli-Goldblatt units (mGU)/ml] mice compared with the wild-type and A1R+/- mice (36.3 +/- 8.5 and 34.1 +/- 9.6 mGU/ml), respectively. The results demonstrate that adenosine acting on A1R is required for TGF and modulates renin release.     

Effects of angiotensin II and angiotensin II antagonist saralasin on cell growth and renin in 3T3 and SV3T3 cells.

Components of the renin-angiotensin system were studied in established cell culture lines of 3T3 and SV3T3 mouse fibroblasts. The renin content in 3T3 cells was significantly higher than in virus-transformed SV3T3 cells. With time after infection, renin decreased in Simian virus 40 transformed cells, while it increased steadily in mock-infected 3T3 cells. In contrast to renin, angiotensinase activity was higher in SV3T3 cells. Angiotensin II stimulated cell proliferation in 3T3 mouse fibroblasts and decreased their renin content in a dose-related manner. In contrast, saralasin, an angiotensin receptor antagonist, inhibited cell growth in 3T3 and SV3T3 cells and caused an increase of cellular renin concentration. The angiotensin fragments angiotensin (2-8) heptapeptide and angiotensin (4-8) pentapeptide had no effect on cell growth. A significant negative correlation was found between cell proliferation and renin levels in 3T3 and SV3T3 cells irrespective of the treatment. Our results indicate (1) that angiotensin II may be involved in cell growth regulation, (2) that a negative feedback exist between angiotensin II added and intracellular renin content, and (3) that virus infection causes a decrease in intracellular renin synthesis, while non-specific angiotensinase activity is increased under this condition.     

Low chloride stimulation of prostaglandin E2 release and cyclooxygenase-2 expression in a mouse macula densa cell line

Reducing luminal NaCl concentration in the macula densa region of the nephron stimulates renin secretion, and this response is blocked by a specific inhibitor of cyclooxygenase-2 (COX-2) (Traynor, T. R., Smart, A., Briggs, J. P., and Schnermann, J. (1999) Am. J. Physiol. Renal Physiol. 277, F706-710). To study whether low NaCl activates COX-2 activity or expression we clonally derived a macula densa cell line (MMDD1 cells) from SV-40 transgenic mice using fluorescence-activated cell sorting of renal tubular cells labeled with segment-specific fluorescent lectins. MMDD1 cells express COX-2, bNOS, NKCC2, and ROMK, but not Tamm-Horsfall protein, and showed rapid (86)Rb(+) uptake that was inhibited by a reduction in NaCl concentration and by bumetanide or furosemide. Isosmotic exposure of MMDD1 cells to low NaCl (60 mm) caused a prompt and time-dependent stimulation of prostaglandin E(2) (PGE(2)) release that was prevented by the COX-2 specific inhibitor NS-398 (10 microm). Reducing NaCl to 60 and 6 mm for 16 h increased COX-2 expression in a chloride-dependent fashion. Low NaCl phosphorylated p38 kinase within 30 min and ERK1/2 kinases within 15 min without changing total MAP kinase levels. Low NaCl-stimulated PGE(2) release and COX-2 expression was inhibited by SB 203580 and PD 98059 (10 microm), inhibitors of p38 and ERK kinase pathways. We conclude that low chloride stimulates PGE(2) release and COX-2 expression in MMDD1 cells through activation of MAP kinases.     

Annexin A1 modulates macula densa function by inhibiting cyclooxygenase 2

Annexin A1 (ANXA1) exerts anti-inflammatory effects through multiple mechanisms including inhibition of prostaglandin synthesis. Once secreted, ANXA1 can bind to G protein-coupled formyl peptide receptors (Fpr) and activate diverse cellular signaling pathways. ANXA1 is known to be expressed in cells of the juxtaglomerular apparatus, but its relation to the expression of cyclooxygenase 2 (COX-2) in thick ascending limb and macula densa cells has not been elucidated. We hypothesized that ANXA1 regulates the biosynthesis of COX-2. ANXA1 abundance in rat kidney macula densa was extensively colocalized with COX-2 (95%). Furosemide, an established stimulus for COX-2 induction, caused enhanced expression of both ANXA1 and COX-2 with maintained colocalization (99%). In ANXA1-deficient mice, COX-2-positive cells were more numerous than in control mice (+107%; normalized to glomerular number; P < 0.05) and renin expression was increased (+566%; normalized to glomerular number; P < 0.05). Cultured macula densa cells transfected with full-length rat ANXA1 revealed downregulation of COX-2 mRNA (-59%; P < 0.05). Similarly, treatment with dexamethasone suppressed COX-2 mRNA in the cells (-49%; P < 0.05), while inducing ANXA1 mRNA (+56%; P < 0.05) and ANXA1 protein secretion. Inhibition of the ANXA-1 receptor Fpr1 with cyclosporin H blunted the effect of dexamethasone on COX-2 expression. These data show that ANXA1 exerts an inhibitory effect on COX-2 expression in the macula densa. ANXA1 may be a novel intrinsic modulator of renal juxtaglomerular regulation by inhibition of PGE(2) synthesis.     

Feedback-mediated dynamics in two coupled nephrons

Previously, we developed a dynamic model for the tubuloglomerular feedback (TGF) system in a single, short-looped nephron of the mammalian kidney. In that model, a semi-linear hyperbolic partial differential equation was used to represent two fundamental processes of solute transport in the nephron’s thick ascending limb (TAL): chloride advection by fluid flow along the TAL lumen and transepithelial chloride transport from the lumen to the interstitium. An empirical function and a time delay were used to relate glomerular filtration rate to the chloride concentration at the macula densa of the TAL. Analysis of the model equations indicated that stable limit-cycle oscillations (LCO) in nephron fluid flow and chloride concentration can emerge for sufficiently large feedback gain magnitude and time delay. In this study, the single-nephron model was extended to two nephrons, which were coupled through their filtration rates. Explicit analytical conditions were obtained for bifurcation loci corresponding to two special cases: (1) identical time delays but differing feedback gains, and (2) identical gains but differing delays. Similar to the case of a single nephron, our analysis indicates that stable LCO can emerge in coupled nephrons for sufficiently large gains and delays. However, these LCO may emerge at lower values of the feedback gain, relative to a single (i.e., uncoupled) nephron, or at shorter delays, provided the delays are sufficiently close. These results suggest that, in vivo, if two nephrons are sufficiently similar, then coupling will tend to increase the likelihood of LCO.     

Comparative physiology of the renin-angiotensin system.

Renin or a renin-like substance is found in the kidneys of many vertebrate species. It is absent from the kidneys of cyclostomes and elasmobranchs and first appears in holosteans and the bony fishes as well as in all higher vertebrate species. Juxtaglomerular cell granules also appear first in holosteans and the bony fishes while the macula densa first appears in amphibians. In telecost fishes, the corpuscles of Stannius contain Bowie-stainable granules and a renin-like pressor substance. Among classes and, in some cases, species of vertebrates, specificity in the reaction of renin with a substrate has been demonstrated. There is also some species and class variation in the angiotensin molecule since angiotensins of fishes, amphibians, reptiles and birds have chemical characteristics different from each other and from those of ammmals. A role for renin in stimulating interrenal gland steroid biosynthesis and in influencing water and ion regulation in nonmammalian vertebrates is discussed.     

A denervated nonfiltering kidney preparation in the rat: a model for study of renin release

To examine the role of the renal vascular receptor in the control of renin secretion in the rat, a denervated, nonfiltering kidney model (DNFK) was developed. The left kidney was subjected to a 2-hr period of total renal ischemia followed by ureteral ligation and section Denervation was accomplished by stripping all visible nerves and painting the renal vessels with 5% phenol. Forty-eight hours later lissamine green dye was injected iv and failed to appear in either the cortical or medullary tubules, indicating that glomerular filtration had ceased. Histological study of these kidneys revealed diffuse tubular necrosis with extensive intratubular cast formation. Norepinephrine content of the DNFK was reduced 91% compared to the contralateral normal kidney (P less than 0.001). In another group of anesthetized rats with a single DNFK, 15 min of suprarenal aortic constriction (SAC) increased plasma renin activity (PRA) from 3.4 +/- 0.6 to 11.5 +/- 1.6 ng AI/ml/hr; in a time control series, PRA was unchanged. To exclude the influence of adrenal catecholamines in this response, bilateral adrenalectomy was performed in a separate group of animals with a DNFK. In this series, SAC also markedly increased PRA. The present data indicate that in the rat the macula densa, the renal nerves, and adrenal catecholamines were not essential for the hyperreninemia induced by a reduction in renal perfusion pressure.     

Dantrolene stimulates renin secretion by rat renal cortical slices but fails to block calcium-dependent inhibition.

Calcium (Ca) is an inhibitory second messenger in renin secretion, and it has been proposed that some first messengers, such as angiotensin II (A-II), antidiuretic hormone (ADH), and N6-cyclohexyladenosine (CHA), increase Ca and thereby inhibit renin secretion by mobilizing Ca from intracellular sequestration sites. The present experiments were designed to test this proposal by using dantrolene, an antagonist of intracellular Ca mobilization. Dantrolene stimulated renin secretion by rat renal cortical slices in a concentration dependent manner; at 0.0, 0.1, and 0.5 mM dantrolene, secretory rates were 8.1 +/- 0.6, 9.4 +/- 0.6 (p less than 0.05), and 14.9 +/- 1.2 (p less than 0.0001) GU/g x hr, respectively. These results could be interpreted to mean that Ca mobilization is occurring at a finite rate during the basal state, and that by antagonizing this process, dantrolene lowers intracellular Ca and thereby stimulates renin secretion. However, 0.1 mM dantrolene failed to antagonize the inhibitory effects on renin secretion of A-II, ADH, and CHA, and only CHA-induced inhibition of renin secretion was antagonized by 0.5 mM dantrolene. We conclude that if A-II, ADH, and CHA inhibit renin secretion by mobilizing Ca from an intracellular storage site, then the site is insensitive to dantrolene.     

Renal response to dietary protein in the house sparrow Passer domesticus

Many birds switch seasonally or during ontogeny between diets of varying protein content. In mammals, high-protein diets induce hypertrophy of the kidney in general and of the thick ascending limbs (TAL) in particular, along with increases in glomerular filtration rate (GFR) and urine flow. A hypothesis to explain these phenomena is that the TAL become increasingly sensitive to peptide hormones (glucagon and antidiuretic hormone [ADH]) released in response to protein feeding; the consequent enhancement of ion reabsorption dilutes urine reaching the macula densa, thereby suppressing tubulo-glomerular feedback (TGF) and causing a rise in GFR. Avian kidneys possess most of the elements involved in this mechanism, including loops of Henle with TAL, sensitivity of TAL to ADH (arginine vasotocin [AVT] in birds), and the elements of TGF. We therefore hypothesized that switching from a low-protein to a high-protein diet would induce responses in birds similar to those found in mammals. We tested this hypothesis by feeding house sparrows, Passer domesticus, isocaloric diets containing either 8% or 30% protein. Birds on high-protein food had larger renal medullae, both in mass and in TAL diameter, but no increase in whole-kidney mass. Urine flow was approximately doubled on high-protein food, but there was no change in GFR. We were not able to detect an increased sensitivity of AVT-induced adenylyl cyclase activity in TAL from high-protein animals, and responsiveness to glucagon was higher in TAL from birds eating low-protein food. We are unable to conclude that a suppression of TGF is responsible for the rise in urine flow in birds eating high-protein foods, and the mechanisms behind the medullary hypertrophy and the diuresis remain to be fully explored.