Inhibition by diltiazem of pressure-induced afferent vasoconstriction in the isolated perfused rat kidney

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Myogenic responses of mouse isolated perfused renal afferent arterioles: effects of salt intake and reduced renal mass

Because defects in renal autoregulation may contribute to renal barotrauma in chronic kidney disease, we tested the hypothesis that the myogenic response is diminished by reduced renal mass. Kidneys from 5/6 nephrectomized mice had only a minor increase in the glomerular sclerosis index. The telemetric mean arterial pressure (108+/-10 mm Hg) was unaffected after 3 months of high-salt intake (6% salt in chow) or reduced renal mass. Afferent arterioles from 5/6 nephrectomized mice and sham-operated controls were perfused ex vivo during step changes in pressure from 40 to 134 mm Hg. Afferent arterioles developed a constriction and a linear increase in active wall tension above a perfusion pressure of 36+/-6 mm Hg, without a plateau. The slope of active wall tension versus perfusion pressure defined the myogenic response, which was similar in sham mice fed normal or high-salt diets for 3 months (2.90+/-0.22 versus 3.22+/-0.40 dynes x cm(-1)/mm Hg; P value not significant). The myogenic response was unaffected after 3 days of reduced renal mass on either salt diet (3.39+/-0.61 versus 4.04+/-0.47 dynes x cm(-1)/mm Hg) but was reduced (P<0.05) in afferent arterioles from reduced renal mass groups fed normal and high salt at 3 months (2.10+/-0.28 and 1.35+/-0.21 dynes x cm(-1)/mm Hg). In conclusion, mouse renal afferent arterioles develop a linear increase in myogenic tone around the range of ambient perfusion pressures. This myogenic response is impaired substantially in the mouse model of prolonged reduced renal mass, especially during high salt intake.

Rho-kinase inhibition reduces pressure-mediated autoregulatory adjustments in afferent arteriolar diameter

Preglomerular resistance is regulated by calcium influx- and mobilization-dependent mechanisms; however, the role of Rho-kinase in calcium sensitization in the intact kidney has not been carefully examined. Experiments were performed to test the hypothesis that Rho-kinase inhibition blunts pressure-mediated afferent arteriolar autoregulatory behavior and vasoconstrictor responses evoked by angiotensin II and P2X1 receptor activation. Rat kidneys were studied in vitro using the blood-perfused juxtamedullary nephron technique. Autoregulatory behavior was assessed before and during Rho-kinase inhibition with Y-27632 (1.0 microM; n = 5). Control diameter averaged 14.3 +/- 0.8 microm and increased to 18.1 +/- 0.9 microm (P < 0.05) during Y-27632 treatment. In the continued presence of Y-27632, reducing perfusion pressure to 65 mmHg slightly increased diameter to 18.7 +/- 1.0 microm. Subsequent pressure increases to 130 and 160 mmHg yielded afferent arteriolar diameters of 17.5 +/- 0.8 and 16.6 +/- 0.6 microm (P < 0.05). This 11% decline in diameter is significantly smaller than the 40% decrease obtained in untreated kidneys. The inhibitory effects of Y-27632 on autoregulatory behavior were concentration dependent. Angiotensin II responses were blunted by Y-27632. Angiotensin II (1.0 nM) reduced afferent diameter by 17 +/- 1% in untreated arterioles and by 6 +/- 2% during exposure to Y-27632. The P2X1 receptor agonist, alpha, beta-methylene ATP, reduced afferent arteriolar diameter by 8 +/- 1% but this response was eliminated during exposure to Y-27632. Western blot analysis confirms expression of the Rho-kinase signaling pathway. Thus, Rho-kinase may be important in pressure-mediated autoregulatory adjustments in preglomerular resistance and responsiveness to angiotensin II and autoregulatory P2X1 receptor agonists.

Effects of amiloride, benzamil, and alterations in extracellular Na+ on the rat afferent arteriole and its myogenic response

Recent studies have implicated epithelial Na+ channels (ENaC) in myogenic signaling. The present study was undertaken to determine if ENaC and/or Na+ entry are involved in the myogenic response of the rat afferent arteriole. Myogenic responses were assessed in the in vitro hydronephrotic kidney model. ENaC expression and membrane potential responses were evaluated with afferent arterioles isolated from normal rat kidneys. Our findings do not support a role of ENaC, in that ENaC channel blockers did not reduce myogenic responses and ENaC expression could not be demonstrated in this vessel. Reducing extracellular Na+ concentration ([Na+]o; 100 mmol/l) did not attenuate myogenic responses, and amiloride had no effect on membrane potential. Benzamil, an inhibitor of ENaC that also blocks Na+/Ca2+ exchange (NCX), potentiated myogenic vasoconstriction. Benzamil and low [Na+]o elicited vasoconstriction; however, these responses were attenuated by diltiazem and were associated with significant membrane depolarization, suggesting a contribution of mechanisms other than a reduction in NCX. Na+ repletion induced a vasodilation in pressurized afferent arterioles preequilibrated in low [Na+]o, a hallmark of NCX, and this response was reduced by 10 micromol/l benzamil. The dilation was eliminated, however, by a combination of benzamil plus ouabain, suggesting an involvement of the electrogenic Na+-K+-ATPase. In concert, these findings refute the premise that ENaC plays a significant role in the rat afferent arteriole and instead suggest that reducing [Na+](o) and/or Na+ entry is coupled to membrane depolarization. The mechanisms underlying these unexpected and paradoxical effects of Na+ are not resolved at the present time.

Effects of inhibition of the Na+/K+/2Cl− cotransporter on myogenic and angiotensin II responses of the rat afferent arteriole

The Na+/K+/2Cl− cotransporter (NKCC) plays diverse roles in the kidney, contributing sodium reabsorption and tubuloglomerular feedback (TGF). However, NKCC is also expressed in smooth muscle and inhibitors of this transporter affect contractility in both vascular and nonvascular smooth muscle. In the present study, we investigated the effects of NKCC inhibitors on vasoconstrictor responses of the renal afferent arteriole using the in vitro perfused hydronephrotic rat kidney. This preparation has no tubules and no TGF, eliminating this potential complication. Furosemide and bumetanide inhibited myogenic responses in a concentration-dependent manner. Bumetanide was ∼20-fold more potent (IC50 1.0 vs. 20 μmol/l). At 100 and 10 μmol/l, furosemide and bumetanide inhibited myogenic responses by 72 ± 4 and 68 ± 5%, respectively. The maximal level of inhibition by bumetanide was not affected by nitric oxide synthase inhibition (100 μmol/l NG-nitro-l-arginine methyl ester). However, the time course for the dilation was slowed (from t1/2 = 4.0 ± 0.5 to 8.3 ± 1.7 min, P = 0.04), suggesting either a partial involvement of NO or a permissive effect of NO on relaxation kinetics. Bumetanide also inhibited ANG II-induced afferent arteriolar vasconstriction at similar concentrations. Finally, NKCC1, but not NKCC2, expression was demonstrated in the afferent arteriole by RT-PCR and the presence of NKCC1 in afferent arteriolar myocytes was confirmed by immunohistochemistry. In concert, these results indicate that NKCC modulation is capable of altering myogenic responses by a mechanism that does not involve TGF and suggest a potential role of NKCC1 in the regulation of vasomotor function in the renal microvasculature.

Differential effects of antihypertensive drugs on renal and glomerular hemodynamics and injury in the chronic nitric-oxide-suppressed rat

BACKGROUND/AIMS

Prolonged nitric oxide synthase (NOS) inhibition with N(omega)-nitro-L-arginine methylester in normotensive and hypertensive rats has been demonstrated to produce severe systemic and glomerular hypertension with glomerular sclerosis, and these changes have become a useful experimental model of hypertensive nephrosclerosis. This review summarizes data from our serial studies as well as work of others who are also investigating the effects of the commonly used antihypertensive drugs (including calcium antagonist, angiotensin-converting enzyme inhibitor, angiotensin II type 1 receptor blocker, aldosterone antagonist and thiazide diuretic) on renal and glomerular hemodynamics, renal function and glomerular histopathology using this model.

METHODS

A Medline search was performed to identify the relevant literature describing renal effects of antihypertensive drugs in models of hypertension and nephrosclerosis produced or exacerbated by NOS inhibition.

RESULTS

Existing data have indicated that most of these drug classes have produced dramatic renoprotective effects, structurally or functionally, on nephrosclerosis induced by prolonged NOS inhibition.

CONCLUSION

This review of experimental studies has provided strong evidence supporting the clinical benefits of antihypertensive drugs for hypertensive patients with renal impairment particularly those with endothelial dysfunction associated with NOS deficiency.

Calcium entry blocker nicardipine inhibits sodium and inorganic phosphate reabsorption independent of renal circulation in dogs

The effects of nicardipine on renal function were studied in anesthetized dogs. The changes in the tubular sodium (Na) and inorganic phosphate (PO(4)) reabsorption caused by the drug infusion into the renal artery without altered systemic and real circulation were especially evaluated. In dogs receiving a smaller dose of nicardipine (5 ng.kg(-1).min(-1)) into the left renal artery the blood pressure and renal circulation did not change, but urine volume and urinary Na and PO(4) excretion increased significantly. In dogs receiving a larger dose of nicardipine (50 ng.kg(-1).min(-1)) into the renal artery, renal plasma flow, urine volume and urinary Na and PO(4) excretion increased significantly, but creatinine clearance did not. The fractional distal Na reabsorption did not change with nicardipine infusion in either group. PO(4) reabsorption is considered to occur mainly in the proximal renal tubule, so its appearance in urine in increased quantities without the changes of systemic and renal circulation suggests proximal activity of the drug.

Role of protein kinase C in Ca channel blocker-induced renal arteriolar dilation in spontaneously hypertensive rats

The present study examined the role of L-/T-type Ca channels and the interaction between these channels and protein kinase C (PKC) in hypertension. The isolated perfused hydronephrotic rat kidney model was used to visualize directly the renal microvascular effects of L-/T-type Ca channel blockers (nifedipine and mibefradil, respectively). Nifedipine reversed the angiotensin II-induced constriction of afferent, but not efferent, arterioles in kidneys from Wistar-Kyoto rats (WKY), and similar magnitude in dilation was observed in spontaneously hypertensive rats (SHR). Although mibefradil elicited dilation of both arterioles, the afferent arteriolar dilation was less in SHR than in WKY (57+/-5% vs. 80+/-4% reversal at 1 micrommol/L). The pretreatment with staurosporine did not alter the angiotensin II-induced afferent arteriolar constriction in WKY, but attenuated this response in SHR. Furthermore, staurosporine enhanced the nifedipine-induced afferent arteriolar dilation (62+/-3% vs. 50+/-3% reversal at 10 nmol/L), and restored the attenuated afferent arteriolar response to mibefradil in SHR. The pretreatment with thapsigargin (a blocker of IP3-mediated intracellular calcium release) prevented the angiotensin II-induced afferent arteriolar constriction in WKY, but caused a significant constriction of afferent arterioles in SHR and efferent arterioles in WKY and SHR; in this setting, mibefradil did not alter efferent arteriolar tone. In conclusion, although both L-type (nifedipine) and T-type Ca channel blockers (mibefradil) exerted potent vasodilation of rat renal microvessels, these actions were modified by PKC, which determined the afferent arteriolar sensitivity to these blockers in SHR. Furthermore, the enhancement in nifedipine-induced afferent arteriolar dilation by staurosporine in SHR suggests that L-type Ca channel activity is augmented in hypertensive animals.

Calcium channel blockade and preservation of renal graft function in cyclosporine-treated recipients: a prospective randomized placebo-controlled 2-year study

BACKGROUND

Studies have provided conflicting results as to the protective role of calcium channel blockers (CCB) in cyclosporine-treated patients with regard to blood pressure control and preservation of renal graft function. Lacidipine is a dihydropyridine CCB that possesses antioxidative, anti-atherosclerotic, and anti-adhesion properties and was shown to prevent cyclosporine-induced nephrotoxicity in a rat model.

METHODS

We conducted a multicenter prospective, randomized, placebo-controlled study in 131 de novo recipients of a cadaveric renal allograft on cyclosporine therapy. The aim of this 2-year study was to assess the effects of lacidipine on graft function (plasma iohexol clearance), renal plasma flow, anastomotic arterial blood flow, deterioration of renal function, blood pressure, acute rejection, and hospitalization rate.

RESULTS

A total of 118 recipients were available for intention-to-treat analysis on efficacy (lacidipine: n=59; placebo: n=59). Graft function assessed by serum creatinine concentration and glomerular filtration rate measured as plasma iohexol clearance, was persistently better in lacidipine-treated patients from 1 year onwards (respectively, P<0.01 and P<0.05). Renal plasma flow and anastomotic blood flow were not significantly higher in lacidipine-treated patients. Three patients on lacidipine therapy and four on placebo experienced treatment failure defined as an increase in serum creatinine from baseline of more than 60% (log-rank test: P=0.57). Study groups did not differ in acute rejection rate, trough blood cyclosporine concentrations, blood pressure, number of antihypertensive drugs, hospitalization rate, and adverse event rate.

CONCLUSIONS

The use of calcium channel blockers in cyclosporine-treated renal recipients results in a significantly better allograft function at 2 years and this effect is independent of blood pressure lowering.

Autoregulation of glomerular filtration rate in patients with type 2 diabetes during isradipine therapy

OBJECTIVE

Calcium-channel blockade impairs renal autoregulation in animals. Impaired renal autoregulation leads to transmission of the systemic blood pressure (BP) into the glomerulus, resulting in capillary hypertension. Information on the impact of calcium antagonist treatment on renal autoregulation in humans is lacking. This study examines the effect of isradipine treatment on the autoregulation of the glomerular filtration rate (GFR).

RESEARCH DESIGN AND METHODS

We performed a randomized double-blind crossover study with 5 mg o.d. isradipine retard and matching placebo in 16 hypertensive patients with type 2 diabetes. Each treatment arm lasted 4 weeks. On the last day of each treatment period, GFR (single-shot 51Cr-EDTA plasma clearance technique for 4 h) was measured twice between 8:00 A.M. and 5:00 P.M., first without clonidine and then after intravenous injection of 75 micro g clonidine. BP was measured every 10 min (Takeda TM2420; A&D, Tokyo).

RESULTS

Clonidine reduced mean arterial BP (MABP) by 15 +/- 1 vs. 11 +/- 1 mmHg (means +/- SE) during placebo and isradipine treatment, respectively (P < 0.05). GFR was reduced from 102 +/- 4 to 99 +/- 4 ml. min(-1). 1.73 m(-2) with placebo (P < 0.01) and from 106 +/- 5 to 98 +/- 5 ml. min(-1). 1.73 m(-2) during treatment with isradipine (P < 0.01). Mean difference (95% CI) between changes in GFR with placebo and isradipine was -4.6 ml. min(-1). 1.73 m(-2) (-10.0 to 0.6) (P = 0.08). Six patients had a reduction in GFR >13% (exceeding the normal limit of autoregulation) combined with a complete pressure-passive vasculature (defined as DeltaMABP% < or = DeltaGFR%) during isradipine treatment versus none during the placebo treatment (P < 0.05).

CONCLUSIONS

Isradipine impairs GFR autoregulation in a sizeable proportion of hypertensive type 2 diabetic patients.

Pharmacologic management of diabetic nephropathy

BACKGROUND

Diabetes mellitus and hypertension are leading causes of end stage renal disease in the United States. Drug therapy that focuses on tight glycemic control and blood pressure control reduces the progression of nephropathy and cardiovascular complications. Angiotensin-converting enzyme (ACE) inhibitors have been shown to reduce the progression of renal disease in patients with diabetes. The angiotensin II receptor blockers (ARBs) losartan and irbesartan have also been shown to reduce microalbuminuria compared with placebo. The nondihydropyridine calcium channel blockers (CCBs) verapamil and diltiazem have been shown to be as effective as an ACE inhibitor in reducing urinary albumin excretion.

OBJECTIVE

This paper reviews the pathophysiology and diagnosis of diabetic nephropathy and recent clinical trials assessing the most appropriate therapeutic options for delaying the progression of nephropathy in patients with diabetes.

METHODS

Primary and review articles that addressed the pathophysiology, diagnosis, and therapeutic options for attenuating the progression of diabetic nephropathy were retrieved through a MEDLINE search (January 1990 to August 2002) and the bibliographies of identified articles were reviewed. English-language sources were searched using the following search terms: diabetes mellitus, nephropathy, proteinuria, ACE inhibitors, and ARBs. Studies published in peer-reviewed journals that were determined to be methodologically sound, with appropriate statistical analysis of the results, were selected for inclusion in this review.

RESULTS

Patients with type 1 diabetes mellitus and evidence of nephropathy should be started on an ACE inhibitor unless contraindicated. The ARBs and ACE inhibitors are viable choices for patients with type 2 diabetes mellitus and evidence of proteinuria. Patients who experience adverse events such as dry cough with ACE inhibitors can be switched to ARBs. Clinical literature suggests that if monotherapy with an ACE inhibitor or ARB does not provide an adequate response, a nondihydropyridine CCB should be added to the regimen. Nondihydropyridine CCBs should also be considered when ACE inhibitors and ARBs are contraindicated.

CONCLUSIONS

ACE inhibitors and ARBs should be considered first-line therapy for patients with type 2 diabetes mellitus and nephropathy. The ACE inhibitors are still the drug of choice for patients with type 1 diabetes mellitus and evidence of incipient or overt nephropathy. If therapeutic goals are not achieved with an ACE inhibitor or ARB, then the addition of a nondihydropyridine CCB should be considered.

Membrane potential controls calcium entry into descending vasa recta pericytes

We tested the hypothesis that constriction of descending vasa recta (DVR) is mediated by voltage-gated calcium entry. K(+) channel blockade with BaCl(2) (1 mM) or TEACl (30 mM) depolarized DVR smooth muscle/pericytes and constricted in vitro-perfused vessels. Pericyte depolarization by 100 mM extracellular KCl constricted DVR and increased pericyte intracellular Ca(2+) ([Ca(2+)](i)). The K(ATP) channel opener pinacidil (10(-7)-10(-4) M) hyperpolarized resting pericytes, repolarized pericytes previously depolarized by ANG II (10(-8) M), and vasodilated DVR. The DVR vasodilator bradykinin (10(-7) M) also reversed ANG II depolarization. The L-type Ca(2+) channel blocker diltiazem vasodilated ANG II (10(-8) M)- or KCl (100 mM)-preconstricted DVR, and the L-type agonist BayK 8644 constricted DVR. The plateau phase of the pericyte [Ca(2+)](i) response to ANG II was inhibited by diltiazem. These data support the conclusion that DVR vasoreactivity is controlled through variation of membrane potential and voltage-gated Ca(2+) entry into the pericyte cytoplasm.

Slowing the progression of renal disease in diabetic patients

OBJECTIVE

To review recent clinical trials that evaluate the most appropriate therapeutic options for delaying the progression of nephropathy in type 2 diabetic patients.

DATA SOURCES

Primary and review articles were retrieved through a MEDLINE search (January 1990-January 2000).

STUDY SELECTION AND DATA EXTRACTION

All studies related to attenuating the progression of nephropathy in diabetic patients were evaluated and included in this review.

DATA SYNTHESIS

Clinical trials with angiotensin-converting enzyme inhibitors (ACEI) have consistently demonstrated a decrease in the progression of renal disease in diabetic patients. The angiotensin-2 receptor blocker (ARB) losartan has been shown to reduce microalbuminuria to the same extent as the ACEI enalapril. The nondihydropyridine calcium-channel blockers (NCCBs) verapamil and diltiazem have also been shown to decrease urinary albumin excretion. Clinical literature suggests that if monotherapy with an ACEI or ARB does not provide an adequate response, an NCCB should be added to the regimen.

CONCLUSIONS

ACEIs should be considered first-line therapy for diabetic patients with nephropathy. ARBs should be considered as an alternative for patients who are unable to tolerate an ACE inhibitor due to adverse effects. If blood pressure goals are not achieved with an ACEI or ARB, then the addition of an NCCB should be considered.

Concerted actions of renal endothelial and macula densa NO systems in the maintenance of extracellular fluid volume

It is now clear that nitric oxide (NO) exerts a substantial influence on renal function and that the kidney has a high capacity to produce NO. However, there are at least two different NO systems in the kidney. The interplay between NO generated by the endothelium and by the macula densa is considered in this review. It seems that endothelial NO increases in response to an increase in perfusion pressure and an increase in distal delivery, whereas macula densa NO decreases upon a sustained increase in distal delivery. Furthermore, evidence is accumulating that macula densa NO may well mediate renin release. Though seemingly in contrast, both the response of the endothelial NO and of the macula densa NO system seem appropriate to restore a perturbation of fluid balance. The function of the tubuloglomerular feedback (TGF) mechanism is likely to be influenced by both sources of NO, because of the close proximity of these NO producing cells to the vascular smooth muscle cells of the afferent arteriole. The endothelial NO system seems to be responsible for short-term, dampening actions to increased afferent arteriolar tone elicited by activation of the TGF system. The macula densa NO system, on the other hand, is probably adapting TGF responses to sustained increases in distal delivery. The analysis presented in this paper is an attempt to integrate the function of the two NO systems into physiological regulation. The exact role of the medullary NOS enzymes remains to be further elucidated.

Effect of nitrendipine on renal function in renal-transplant patients treated with cyclosporin: a randomised trial

BACKGROUND

Calcium antagonists such as nitrendipine reduce the effects of cyclosporin on renal haemodynamics, however, their long-term efficacy has not been established. We did a randomised trial to investigate the effects of nitrendipine on renal function in renal-transplant patients treated with cyclosporin.

METHODS

253 renal-transplant patients were recruited: 52 normotensive patients (diastolic blood pressure <90 mm Hg) were assigned placebo and 57 nitrendipine 5 mg twice daily; 71 hypertensive patients (diastolic blood pressure >90 to <115 mm Hg) were assigned placebo and 73 nitrendipine 10 mg twice daily. Nitrendipine was increased to 20 mg twice daily if the target diastolic blood pressure (<90 mm Hg) was not achieved. The patients were seen once a month for 24 months; blood pressure and serum creatinine concentration were recorded at each visit. Analysis was by intention to treat.

FINDINGS

63 patients were withdrawn (35 nitrendipine, 28 placebo). The mean serum creatinine concentration at baseline was slightly higher in the nitrendipine group (146.7 micromol/L [SE 4.42]) than in the placebo group (137.0 micromol/L [3.54]. At the 24-month endpoint or at dropout, serum creatinine concentration was significantly higher in the 123 patients in the placebo group than the 130 patients in the nitrendipine group (160.8 [7.1] vs 148.5 [5.3], p for effect of treatment=0.025, analysis of covariance in a two-way classification; 95% CI for difference -1.77 to -22.98). At study entry, the blood pressures of the placebo and the nitrendipine groups were almost identical. At 24 months, blood pressure was higher in the normotensive patients given a placebo than in those patients given nitrendipine. By contrast, blood-pressure values were similar in those hypertensive patients given a placebo and those given nitrendipine at the end of treatment.

INTERPRETATION

The calcium antagonist nitrendipine has no adverse effects on kidney function in renal-transplant patients with cyclosporin. The drug has a small but significant nephroprotective effect, that is independent of the drug's antihypertensive action.

Renal endothelial and macula densa NOS: integrated response to changes in extracellular fluid volume

If, only 20 years ago, anyone had postulated that the absence of nitric oxide gas (NO) would lead to severe hypertension and destruction of the vascular bed of the kidney within weeks, it is not unlikely that smiles of pity would have appeared on the faces of fellow researchers. By now, this has become common knowledge, and hundreds of reports have appeared on the regulation of vascular and renal function by nitric oxide. The amount of information complicates the design of a concept on how NO participates in control of extracellular fluid volume (ECFV) by the kidney. This review analyzes the function of endothelial and macula densa NO synthase (NOS) in the regulation of renal function. From this analysis, endothelial NOS (eNOS)-derived NO is considered a modulator of vascular responses and of renal autoregulation in particular. Increases in renal perfusion pressure and sodium loading will increase eNOS activity, resulting in vasodilatation and depression of tubuloglomerular feedback system responsiveness. Endothelium-derived NO seems important to buffer minute-to-minute variations in perfusion pressure and rapid changes in ANG II activity. In contrast, macula densa NOS is proposed to drive adaptations to long-term changes in distal delivery and is considered a mediator of renin formation. Increases in perfusion pressure and distal delivery will depress the activity and expression of the enzyme that coincides with, and possibly mediates, diminished renin activity. Together, the opposite responses of eNOS and macula densa NOS-derived NO to changes in ECFV lead to an appropriate response to restore sodium balance. The concept that the two enzymes with different localizations in the kidney and in the cell are producing the same product, displaying contrasting responses to the same stimulus but nevertheless exhibiting an integrated response to perturbation of the most important regulated variable by the kidney, i.e., the ECFV, may be applicable to other tissues.

Antihypertensive treatment of patients with proteinuric renal diseases: risks or benefits of calcium channel blockers?

In patients with proteinuric renal diseases the rate of progression of renal insufficiency is determined by the level of blood pressure and proteinuria. It has been demonstrated that strict blood pressure control with angiotensin converting enzyme (ACE)-inhibitors or beta-blockers, aimed at reaching values below 130/80 mm Hg, attenuates the deterioration of renal function. In general, the beneficial effects of these drugs are reflected in a parallel lowering of proteinuria. Calcium channel blockers are effective antihypertensive drugs, however, their safety in patients with proteinuric renal diseases and renal insufficiency may be questioned because of reported untoward effects on urinary protein excretion. The present review discusses the potential benefits and risks of calcium channel blockers (CCBs) in the treatment of patients with renal diseases. To this end we have evaluated the effects of these drugs in animal models of progressive renal injury. In these animal models adverse effects of CCBs have been reported which are attributed to an impairment of autoregulation. In patients with proteinuria, the dihydropyridine CCBs do not lower proteinuria despite a reduction of blood pressure. Studies on the effects on the course of renal function are limited, however, the available data do suggest that this class of CCBs may be less advantageous than other antihypertensive drugs, thus arguing against the use of these agents as first-line drugs in patients with proteinuric renal diseases. Information on the effects of the non-dihydropyridine CCBs is limited to a small number of studies in patients with diabetic renal disease. Although the data suggest that these classes of CCBs might be more beneficial, more studies are needed, particularly in patients with non-diabetic renal diseases, before founded conclusions can be reached.

Integrating multiple paracrine regulators of renal microvascular dynamics

There has been tremendous growth in our knowledge about the multiple interacting mechanisms that regulate renal microvascular function. Paracrine signals originating from endothelial and epithelial cells exert profound influences on the basal tone and reactivity of the pre- and postglomerular arterioles. Selective responsiveness of these arterioles to various stimuli is possible because of differential activating mechanisms in vascular smooth muscle cells of afferent and efferent arterioles. Afferent arterioles rely predominantly on voltage-dependent calcium channels, while efferent arterioles utilize other mechanisms for calcium entry as well as intracellular calcium mobilization. The autoregulatory responses of preglomerular arterioles exemplify the selectivity of these complex control mechanisms. The myogenic mechanism responds to increases in renal perfusion pressure through "stretch-activated" cation channels that lead to depolarization, calcium entry, and vascular contraction. Autoregulatory efficiency is enhanced by the tubuloglomerular feedback (TGF) mechanism which responds to flow-dependent changes in tubular fluid composition at the level of the macula densa and transmits signals to the afferent arterioles to alter the activation state of voltage-dependent calcium channels. Recent studies have implicated extracellular ATP as one paracrine factor mediating TGF and autoregulatory related signals to the afferent arterioles. Other paracrine agents including nitric oxide, angiotensin II, adenosine, and arachidonic acid metabolites modulate vascular responsiveness in order to maintain an optimal balance between the metabolically determined reabsorptive capabilities of the tubules and the hemodynamically dependent filtered load.

Functional impairment of renal afferent arteriolar voltage-gated calcium channels in rats with diabetes mellitus

Experiments were performed to test the hypothesis that diabetes mellitus is associated with impaired afferent arteriolar responsiveness to opening of voltage-gated calcium channels. Diabetes was induced by injection of streptozocin (65 mg/kg, i.v.) and insulin was administered via an osmotic minipump to achieve moderate hyperglycemia. Sham rats received vehicle treatments. 2 wk later, the in vitro blood-perfused juxtamedullary nephron technique was used to allow videomicroscopic measurement of afferent arteriolar contractile responses to increasing bath concentrations of either Bay K 8644 or K+. Baseline afferent arteriolar diameter in kidneys from diabetic rats (26.4+/-1.2 microm) exceeded that of Sham rats (19.7+/-1.0 microm). Bay K 8644 evoked concentration-dependent reductions in afferent diameter in both groups of kidneys; however, arterioles from Sham rats responded to 1 nM Bay K 8644 while 100 nM Bay K 8644 was required to contract arterioles from diabetic rats. The EC50 for K+-induced reductions in afferent arteriolar diameter was greater in diabetic kidneys (40+/-4 mM) than in kidneys from Sham rats (28+/-4 mM; P < 0.05). In afferent arterioles isolated by microdissection from Sham rats and loaded with fura 2, increasing bath [K+] from 5 to 40 mM evoked a 98+/-12 nM increase in intracellular Ca2+ concentration ([Ca2+]i). [Ca2+]i responses to 40 mM K+ were suppressed in afferent arterioles from diabetic rats (delta = 63+/-5 nM), but were normalized by decreasing bath glucose concentration from 20 to 5 mM. These observations indicate that the early stage of insulin-dependent diabetes mellitus is associated with a functional defect in afferent arteriolar L-type calcium channels, an effect which may contribute to suppressed afferent arteriolar vasoconstrictor responsiveness and promote glomerular hyperfiltration.

Renal vascular responses to high and low ionized calcium: influence of norepinephrine in the isolated perfused rat kidney

OBJECTIVE AND DESIGN

The aim of this study was to examine the influence of norepinephrine (NE) on renal vascular responses to high (1.88 mmol/L) and low (0.56 mmol/L) perfusate-ionized calcium ([Ca2+]) in the isolated perfused kidney of the rat. High and low [Ca2+] encompassed the clinical concentration range in this multiexperiment, randomized trial.

MATERIALS AND METHODS

Rats (n = 25), ranging in age from 3 to 4 months, were anesthetized and the ureter and renal artery were cannulated. The right kidney was perfused with oxygenated, warmed albumin (67 g/L) containing Krebs-Henseleit buffer and placed in a thermostated chamber without interruption of flow. In protocol A (n = 7), steady-state high [Ca2+] (1.88 mmol/L) and low [Ca2+] (0.56 mmol/L) were instituted in randomized order in each experiment under basal conditions. In protocol B (n = 9), the same interventions were instituted during constant rate NE infusion. Changes in renal flow were measured at constant perfusion pressure (110 mm Hg), and renal vascular resistance (RVR) was calculated. Renal function was assessed by clearance of [14C]inulin and by fractional excretion of sodium. With NE-induced preconstriction, the increase in RVR observed during high [Ca2+] was +17.8 +/- 1.8% of control, and the decrease in RVR observed during low [Ca2+] was -35.9 +/- 8.2% of control. Both values were greater by a factor of 2 than corresponding results obtained under basal conditions (7 +/- 2.1% vs. -13.5 +/- 4.1% of control, respectively, p < 0.05). Whereas the decrease in glomerular filtration rate with high [Ca2+] was not significantly influenced by NE pretreatment (-9 +/- 1.8% of control with high [Ca2+] in combination with NE vs. 4.1 +/- 0.7% of control under basal conditions), the increase in glomerular filtration rate with low [Ca2+] was significantly greater in the presence of NE (12 +/- 0.7 vs. 102 +/- 8.5% of control, p < 0.01).

CONCLUSIONS

Whereas under basal conditions renal vascular effects of high and low [Ca2+] (varied within the clinical concentration range) are small, the changes recorded with the same interventions after NE pretreatment are increased by a factor of > 2. Hypercalcemia-induced renovascular constriction and decreased function are unfavorable, especially in patients who are at risk for renal dysfunction from other causes.

Pressure-induced contraction of the juxtamedullary afferent arterioles in spontaneously hypertensive rats

1. In the SHR juxtamedullary nephron preparation, the increase of the perfusion pressure from 80 to 160 mmHg increased the diameters of arcuate arteries but produced a pressure-dependent contraction of the afferent arterioles, a response that can account for renal autoregulation. 2. The pressure-induced contractions of the afferent arterioles were abolished by 1 microM nifedipine and by 10 microM furosemide, suggesting that the autoregulatory responses are mainly mediated by tubuloglomerular mechanisms and can be abolished by calcium antagonists.

Effect of elevated extracellular glucose concentrations on transmembrane calcium ion fluxes in cultured rat VSMC

Blood flow autoregulation is impaired in early diabetes mellitus, predisposing the renal microcirculation to injury. These hemodynamic changes have been strongly implicated in the development and progression of diabetic glomerulopathy. Blood flow autoregulation is predominantly a myogenic reflex which is strongly dependent on Ca2+ uptake by vascular smooth muscle cells (VSMC). Because impaired blood flow autoregulation may be responsive to glycemic control, the present study examined the effects of elevated extracellular glucose concentrations on basal, voltage sensitive and receptor operated Ca2+ uptake by VSMC. Confluent cultured rat VSMC were exposed to: (1) control medium (CM; 5 mM glucose); (2) high glucose medium (HGM; 10 to 30 mM glucose); or (3) osmotic control medium (OCM; glucose 5 mM + L-glucose 25 mM or mannitol 25 mM). A threshold glucose concentration of 15 mM markedly and maximally depressed basal Ca2+ uptake by VSMC (HGM 52% vs. CM). In addition, HGM significantly depressed voltage sensitive Ca2+ uptake by VSMC as determined by responses to BAY K 8644 (10(-7) M) or high extracellular [K+] (65 mM, HGM 50% vs. CM). HGM similarly depressed pressor hormone-stimulated Ca2+ uptake (AVP or Ang II 10(-7) M) by VSMC. The effects of HGM on Ca2+ uptake were time exposure dependent and reversible. Ca2+ uptake by VSMC in the presence of OCM did not differ from CM. Elevated extracellular glucose concentrations thus exert a direct and profound effect on basal, voltage sensitive and receptor operated Ca2+ uptake by VSMC. These observations may provide a biochemical basis for glucose-induced dysregulation of regional blood flow autoregulation in early diabetes mellitus.

Isolated rat glomerular cells demonstrate L-type Ca(2+)-channel activity

The presence of L-type calcium (Ca2+)-channels and the effects of Ca(2+)-channel antagonists on cells of rat glomeruli were investigated. Glomeruli were isolated by graded sieving and after preincubation (10 min) in zero Ca2+, the uptake of 45Ca2+ by glomerular cells was measured. Depolarization with KCl (50 mM) or the dihydropyridine agonist Bay K 8644 (10 microM) stimulated 45Ca2+ uptake by 13% and 24%, respectively, above control (100%), which was inhibited by nifedipine (Nif, 10 microM), P < 0.05, and by both S and R isomers of verapamil (Ver, 10 microM), P < 0.001. In a separate experimental preparation, isolated glomeruli were preloaded (45 min) with 45Ca2+. Following a 45 min perifusion (37 degrees C, CaCl2 1.26 mM, in the absence of 45Ca2+), both KCl (50 mM) and Bay K 8644 (10 microM) induced cellular 45Ca2+ efflux with peak values above control of 11% and 15%, respectively, (P < 0.05). Exposure to Bay K 8644 preceded by depolarization with KCl resulted in enhanced 45Ca2+ efflux identifying the presence of voltage-dependent Ca(2+)-channel activity. Cultured rat mesangial cells grown to confluence on coverslips were preloaded with Fura-2 and cytosolic Ca2+ was measured by microfluorometry. KCl (50 mM), gramicidin (2 microM) and/or Bay K 8644 (6 microM) stimulated Ca2+ influx which was inhibited by Ver (10 microM). Ver did not alter endothelin-stimulated Ca2+ signalling. We conclude that L-type Ca2+ channels are present on both rat glomerular (endothelial and/or mesangial) cells in vivo and on cultured mesangial cells, and their activation may be hormone specific.

Treatment of arterial hypertension in diabetic humans: importance of therapeutic selection

This study was undertaken to test the hypothesis that, given equal arterial pressure reductions, the combination of an angiotensin converting enzyme (ACE) inhibitor and calcium antagonist slows declines in renal function and yields greater reductions in albuminuria over either agent alone. This hypothesis was evaluated in four groups of hypertensive, non-insulin dependent, diabetic subjects with renal insufficiency (N = 30). Renal hemodynamics, albuminuria and metabolic parameters were evaluated for a period of one year. Subjects were all placed on a 90 mEq sodium, 0.8 g/kg protein, 1500 calorie American Diabetes Association diet for the entire length of the study. Subjects were followed for two weeks off antihypertensive medications and were subsequently randomized to either lisinopril, alone (group I), sustained release verapamil, alone (group II), reduced doses of both lisinopril and sustained release verapamil (group III), and hydrochlorothiazide with guanfacine (group IV). At the end of one year group III had the greatest reduction in albuminuria (78 +/- 7%, group III vs. 59% +/- 4, group I: P less than 0.05). In addition, the decline in glomerular filtration rate (GFR) was the lowest in this group (0.28 +/- 0.07, group III vs. 0.69 +/- 0.12, group I; P less than 0.05) although there was no significant difference between groups II and IV. The highest side effect profiles were noted in group IV, the least in group III. The greatest reductions in renal hemodynamics occurred in all groups within the first month; however, striking differences between groups were noted (7.4 +/- 2%, group I vs. 1.4 +/- 2%, group III; P less than 0.05). We conclude that the combination of reduced doses of an ACE inhibitor and calcium antagonist attenuate both albuminuria and the rate of decline in glomerular filtration rate. Furthermore, the combination of these classes of agents appear to yield the lowest side effect profile over either agent alone. Lastly, high doses of ACE inhibition alone may be detrimental to renal function in late stage diabetics with renal insufficiency.

The effect of two different calcium antagonists on the glomerular haemodynamics in the dog

Kidney function of beagles fed a constant amount of food containing 3 mmol sodium.kgbodywt-1.day-1, and anaesthetized with pentobarbitone was investigated by clearance and micropuncture techniques during an intrarenal infusion of saline or the calcium antagonists verapamil (VER, 4 micrograms.kgbodywt-1.min-1) or nifedipine (NIF, 0.3 microgram.kgbodywt-1.min-1). Neither drug changed the mean arterial pressure. Apart from the natriuresis and diuresis, which were significantly greater with NIF than with VER, the response to both drugs was similar. Increases in renal blood flow (RBF; 17% with VER, 20% with NIF), glomerular filtration rate (GFR; VER: 34%; NIF: 39%) and filtration fraction (VER: 12%; NIF: 14%) were observed; similar values were obtained at the single nephron level. Pressure in glomerular capillaries, measured directly after ablation of a thin layer of cortex corticis, was increased by 11% with VER and 10% with NIF; no changes in proximal tubular and peritubular capillary pressure were seen. The glomerular ultrafiltration coefficient (Kf) did not change with either drug. Total arteriolar resistance was decreased (VER: 20%; NIF: 15%) due to a decrease in afferent resistance (VER: 31%; NIF: 27%) with no corresponding change in efferent resistance. The cause of the lack of responsiveness of the efferent arteriole remains unclear. In conclusion, in acute experiments with intrarenal administration, both drugs increase RBF and GFR by a preferential afferent dilatory mechanism without any change in Kf.

Felodipine in the treatment of patients with severe hypertension and impaired renal function

Twenty-three patients with severe hypertension and impaired renal function were included in an open study of the efficacy and tolerance of felodipine treatment over 6 months. All patients were previously treated with a diuretic, a beta blocker, and a vasodilator, and eight of them also received an ACE inhibitor. At the start of felodipine treatment the previously used vasodilator was withdrawn. In nine patients the concomitant antihypertensive treatment was reduced during the study. The glomerular filtration rate (GFR), as 51Cr EDTA clearance, was determined before and at the end of the study. The blood pressure (BP) and heart rate (HR) were recorded at all clinical visits in the morning 12 hours after the evening dose of felodipine and 2 hours after the morning dose. Plasma concentrations of felodipine were measured at every visit before the morning dose and 2 hours after dose. The BP was reduced after felodipine was substituted for the previously used vasodilator. A significant additional anti-hypertensive effect was recorded 2 hours after the dose and amounted to -37 +/- 22/-15 +/- 12 mmHg (p = 0.0001/p = 0.0002) at 6 months. The effect measured 12 hours after the dose was less pronounced and was -11 +/- 28/-6 +/- 10 mmHg (p = 0.15/p = 0.03). Mean GFR was unchanged during the study, 38 +/- 19 versus 38 +/- 19 ml/min (n = 16). There was a sixfold interindividual variation in the trough plasma concentrations at steady state at the same drug dosage. Higher plasma concentrations seemed to be required to achieve the same antihypertensive effects as in patients with less severe hypertension and normal renal function.(ABSTRACT TRUNCATED AT 250 WORDS)

Pressure-induced vasoconstriction of renal microvessels in normotensive and hypertensive rats. Studies in the isolated perfused hydronephrotic kidney

The capacity of small arteries to respond to increased intravascular pressure may be altered in hypertension. In the kidney, hypertension is associated with a compensatory shift in the autoregulatory response to pressure. To directly determine the effects of established hypertension on the renal microvascular response to changes of perfusion pressure, we evaluated pressure-induced vasoconstriction in hydronephrotic kidneys isolated from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Vessel diameters of interlobular arteries (ILAs) and afferent and efferent arterioles were determined by computer-assisted videomicroscopy during alterations in renal arterial pressure (RAP) from 80 to 180 mm Hg. Increased RAP induced a pressure-dependent vasoconstriction in preglomerular vessels (afferent arterioles and ILAs), but not in postglomerular vessels (efferent arterioles). The calcium antagonist nifedipine prevented pressure-induced afferent arteriolar vasoconstriction with a similar half-maximal inhibitory concentration (IC50) (WKY, 63 +/- 27 vs. SHR, 60 +/- 32 nM). The pressure-activation curves for ILAs in SHR and WKY were similar. In contrast, the pressure-activation curve for afferent arterioles in SHR kidneys exhibited a rightward shift, which was observed at every segment of the afferent arteriole (i.e., near ILA, at midportion, and near glomerulus). These findings demonstrate that the ILA and the afferent arteriole both possess the ability to constrict in response to increased pressure, whereas this property is lacking in the efferent arteriole. Hypertension was associated with a compensatory shift in the pressure response of the afferent arteriole, such that higher RAPs were required to elicit vasoconstriction in this vessel.

Effects of amlodipine on renal hemodynamics

Recently, attention has focused on the effects of calcium antagonists on renal function. When administered in vitro to the isolated perfused kidney, calcium antagonists exhibit consistent actions permitting characterization of their renal effects. Calcium antagonists do not affect the vasodilated isolated perfused kidney, but they do dramatically alter the response of the kidney to vasoconstrictor agents. This study examined the effects of the novel dihydropyridine amlodipine on the hemodynamic response of the isolated perfused kidney to angiotensin II. Amlodipine completely reversed the angiotensin II-induced decrement in glomerular filtration rate of this model (0.72 +/- 0.15, 0.26 +/- 0.10 and 0.73 +/- 0.12 ml/min/g for control, angiotensin II and angiotensin II plus 0.1 microM amlodipine respectively). In contrast, amlodipine only partially restored renal perfusate flow (35.8 +/- 2.7, 14.7 +/- 1.9 and 23.7 +/- 2.5 ml/min/g for control, angiotensin II and angiotensin II plus amlodipine), thereby increasing filtration fraction. These findings are consistent with previous observations from this laboratory indicating that dihydropyridines predominantly vasodilate preglomerular renal resistance vessels and through this mechanism exert a preferential augmentation of glomerular filtration rate.

Renal hemodynamic effects of calcium antagonists

Recently, attention has focused on the effects of calcium antagonists on renal function. When administered in vitro to the isolated perfused kidney, calcium antagonists exhibit predictable actions allowing for characterization of their renal effects. Calcium antagonists do not affect the vasodilated isolated perfused kidney; however, they do dramatically alter the response of the kidney to vasoconstrictor agents. In the presence of norepinephrine, calcium antagonists markedly augment the glomerular filtration rate but produce only a modest improvement in renal perfusion. A study using the postischemic hydronephrotic rat kidney model that permits direct visualization of afferent and efferent arterioles, this study demonstrated that this preferential augmentation of the glomerular filtration rate is primarily attributable to a selective vasodilation of pre-glomerular vessels. Although the clinical implications of such observations are not yet clear, preliminary studies in experimental animal models indicate that calcium antagonists might exert salutary effects on renal function in clinical settings characterized by impaired renal hemodynamics. The possible benefits of calcium antagonists in ameliorating the development of renal dysfunction in patients in whom there is increased risk of acute renal insufficiency remain to be evaluated.

Calcium antagonists and acute renal failure

PURPOSE

To critically review the question of whether calcium antagonists can prevent or attenuate post-ischemic acute renal failure (ARF).

PATIENTS AND METHODS

Using a computer-assisted search, we identified all experimental and clinical studies published in English between 1980 and 1988 in which the main research question addressed the efficacy of verapamil, diltiazem, or nifedipine in the treatment of post-ischemic ARF. Studies were then selected for review based on clearly specified inclusion criteria and evaluated against accepted methodologic guidelines.

RESULTS

In experimental studies of warm renal ischemia, calcium antagonists provided significant protection of glomerular filtration rate (GFR) when given before and after ischemic injury; however, isolated pre- or post-ischemic treatment produced equivocal results. In autotransplantation studies on protracted cold ischemia, verapamil produced a modest, although physiologically significant, increase in GFR during warm reperfusion but failed to alter graft survival. Studies of calcium antagonists in humans with ARF have been confined to patients undergoing renal transplantation. Diltiazem given both to donor grafts and to recipients produced a significant reduction in the rate of delayed graft function but failed to improve one-year graft survival. This result may be due to the study of small numbers of low-risk patients. At present, there is no conclusive evidence that one calcium antagonist is more efficacious than another in the treatment of post-ischemic ARF.

CONCLUSIONS

Calcium antagonists appear to prevent or reduce the severity of post-ischemic ARF only when given prior to and after the ischemic insult. As a result, these agents will have most utility in the setting of renal transplantation. Although the efficacy of these agents in reducing delayed graft function appears to be established, their failure to improve graft survival is poorly understood and requires further investigation.

Emerging issues in the cellular biology of the cardiovascular system

Current theory suggests that life began in a prebiologic era, progressed to a ribonucleic-deoxyribonucleic cellular era, and finally entered an era characterized by multicellular organisms. If this progression is correct, it is not surprising that, as medicine studies living organisms with increasing sophistication, factors that are initially discovered to have systemic effects are, in many instances, later determined to have paracrine, autocrine or even intracellular ("intracrine") effects. This schema is potentially of value in analyzing the pathogenesis of cardiovascular disease and, in particular, the development of the sequelae of hypertension. A case is made for the idea that the actions of common peptide and nonpeptide factors at local tissue levels can play an important role in the development of atherosclerosis and left ventricular hypertrophy. In making this case, the potential roles of insulin, angiotensin II and other vasoactive factors are considered. In addition, it is argued that some peptide and nonpeptide factors with cardiovascular impact may operate in the intracellular environment, thus broadening prospects for study and intervention. Finally, genomic alterations either spontaneously occurring or resulting from chronic stimulation or viral infection are considered and their potential role is discussed.

Function of the hypertensive kidney during calcium flux manipulation

Dihydropyridine calcium channel agonists and antagonists elicit exaggerated glomerular and circulatory responses from kidneys isolated from Dahl rats genetically programmed to develop NaCl-induced hypertension (Dahl S rats). These differential responses are further magnified by NaCl loading. In contrast, "chemical sympathectomy" with 6-hydroxydopamine enhances renal vascular responses to calcium channel agonists in a manner that depends on the antecedent dietary NaCl intake, and is independent of genetic predilection to develop NaCl-induced hypertension. These findings are consistent with the hypothesis that aberrations of vascular and perhaps glomerular calcium entry modulation may be determinants of altered renal hemodynamics in NaCl-sensitive hypertension. The latter may be responsible for the enhanced responsiveness to calcium channel antagonists observed in NaCl-sensitive hypertension in humans.

Calcium antagonists and the renal hemodynamic response to vasoconstrictors

These findings demonstrate that calcium antagonists reverse renal vasoconstriction in a variety of settings. The ability of calcium antagonists to augment GFR of the vasoconstricted kidney is striking and has also been demonstrated in a number of in vivo settings. These observations and others raise the possibility that calcium antagonists have potential utility in the treatment of a number of disorders characterized by renal ischemia and consequent renal insufficiency. Further studies to evaluate this possibility are required. The unique effects of calcium antagonists on GFR reflect a regional heterogeneity within the renal microcirculation and a preferential action of calcium antagonists on the afferent arteriole. Final resolution of the pharmacological basis for the renal hemodynamic actions of calcium antagonists will require a more complete understanding of the divergent activating mechanisms within the renal microcirculation.

Calcium channel antagonists, Part I: Fundamental properties: mechanisms, classification, sites of action

Ca2+ channel antagonists are agents that interact with the voltage-dependent Ca2+ channel in a highly specific way. The prototype agents of cardiovascular importance are verapamil, nifedipine, and diltiazem, in historical order of appearance. These agents all have different molecular structures and bind separately with receptor sites located in or near the calcium channel, at molecular sites still to be fully identified. There are probably three distinct receptor sites (V, N, D) which stand in relation to the "gate" of the long-acting "L" calcium channel. There is probably overlap among the receptor sites, especially between the V and D sites to explain their common properties. All three agents inhibit the voltage-dependent calcium channel in vascular smooth muscle and also myocardial slow calcium channels. The ratio of the arterial to the myocardial effect is an index of the arterial selectivity, generally held to be a desirable property because the negative inotropic effect is usually a liability. The general clinical impression that nifedipine is the agent most active in vascular tissue in relation to the myocardial effect is supported by data on the relative potencies of these three agents on blood perfused dog preparations and by a comparison of the potency on rat vascular (portal vein) versus myocardial effects. Nonetheless all three agents are highly active in the inhibition of K(+)-induced vascular contractions (nifedipine 10(-9) M to 10(-8) M; verapamil 10(-7) M to 10(-6) M; and diltiazem 5 x 10(-7) M to 10(-6) M; concentrations for 50% inhibition of K(+)-induced vascular contractions in rat or rabbit aorta; comparative data for resistance vessels not available). The clinical impression that verapamil and diltiazem are more active on nodal tissue is also supported by a comparison of potencies on blood perfused dog nodal preparations in comparison with effects on coronary flow, with verapamil and diltiazem being approximately 10x more potent on the AV node than increasing coronary blood flow, so that the nodal effect is first detected. These basic pharmacological properties explain why all these three agents have clinical effects relevant to inhibition of vascular contraction (antihypertensive and antianginal effects) and only verapamil and diltiazem have clinically relevant inhibitory effects on the AV node (inhibition of supraventricular tachycardias). The comparative potencies of verapamil, diltiazem, and nifedipine in angina and hypertension will be examined in Parts II and III of this review.

Renal hemodynamic effects of calcium antagonists

Although the cardiovascular actions of calcium antagonists have been studied extensively, it is only recently that attention has focused on the effects of calcium antagonists on renal function. Variable actions of calcium antagonists on renal hemodynamics have been observed when these agents are used in experimental animal models. In contrast, when administered in vitro to the isolated perfused kidney, calcium antagonists demonstrate more predictable actions. Calcium antagonists do not affect the vasodilated isolated kidney, but they do dramatically alter the response of the kidney to vasoconstrictor agents. In the presence of norepinephrine, calcium antagonists produce a modest improvement in renal perfusion and significantly augment glomerular filtration rate. This preferential augmentation of glomerular filtration rate may be attributable to a selective vasodilation of pre-glomerular vessels. Although the clinical implications of such observations are not yet clear, preliminary studies indicate that calcium antagonists may exert salutary effects on renal function in clinical settings that are characterized by impaired renal hemodynamics. The possible benefits of calcium antagonists in ameliorating the development of renal dysfunction in patients in whom there is increased risk for the development of acute renal insufficiency remain to be evaluated.