Bernward Garthoff

Interference of the calcium antagonist nisoldipine with the abnormal response of vessels from hypertensive rats to alpha-adrenergic stimulation.

The effects of the calcium antagonist nisoldipine on contractions stimulated by phenylephrine and B-HT 920 (agonists of α1- and α2-adrenoceptors) in isolated aortic rings from stroke-prone spontaneously hypertensive rats (SHRSP) and from normotensive Wistar-Kyoto rats (WKY) were investigated in vitro. Phenylephrine and B-HT 920 produced concentration-dependent contractions of vessels from both groups of animals. The absolute force of the contractions was less in the aortae from hypertensive rats after all doses of both agonists. Nisoldipine inhibited the B-HT 920-induced contraction much more in vessels from SHRSP than in those from normotensive WKY rats (IC50 = 1.5 x 10 10 versus 7 x 10-9 g/ml). The phenylephrine contractions were inhibited in SHRSP aortae by higher concentrations (IC50 −8.5 × 10 8 g/ml) of nisoldipine: in WKY, nisoldipine only produced a slight inhibition of phenylephrine-induced contractions. The inhibitory concentrations of nisoldipine on BHT-920-induced contractions are similar to those for the inhibition of the calcium or depolarization-induced contractions in other experiments. The α2-agonist-induced contractions of rat aorta are dependent on trans-membrane calcium supply. The higher efficacy of nisoldipine in aortae from SHRSP suggests an increased trans-membrane availability of calcium ions in hypertension.

Mode of antihypertensive action of nitrendipine.

Summary The antihypertensive effect of nitrendipine cannot be explained only by its reduction of the increased peripheral vascular resistance. In contrast to the antihypertensive vasodilators, nitrendipine improves impaired renal function and prevents generalized vasculopathy in hypertensive animals. Chronic treatment with nitrendipine prevents spontaneous (Okamoto rats) and salt-induced (Dahl rats) hypertension and cardiac hypertrophy. In rats with established hypertension, nitrendipine normalizes blood pressure, reduces cardiac hypertrophy, and improves renal ischaemia. In salt-induced malignant hypertension in stroke-prone spontaneously hypertensive rats, nitrendipine only slightly reduces blood pressure but dramatically improves survival and prevents vascular lesions in the heart, brain, andkidneys. Nitrendipine reduces the intracellular availability of calcium ions in vascular smooth muscle responsible for the increased peripheral and renovascular resistance in hypertension. Moreover, in preventing the deleterious calcium overload, nitrendipine preserves tissue integrity and increases life span in malignant hypertension.

Adequate substitution with electrolytes in toxicological testing of “loop” diuretics in the dog☆

Abstract In long-term toxicologic experiments on dogs, muzolimine was administered orally in doses of 0.3, 1.2, or 4.8 mg/kg body weight for 13 weeks; 0.3, 1, 3, or 10 mg/kg for 52 weeks with electrolytes substituted in drinking water; or in doses of 1.2 or 4.8 mg/kg for 13 weeks with full electrolyte substitution by food. In a supplementary study, muzolimine was given after a chloride-poor diet. Experiments without electrolyte substitution seemed to indicate dose-dependent increases in blood urea and blood creatinine and characteristic kidney alterations (e.g., pale and enlarged kidneys, subcapsular cysts, widened tubules, tubular atrophy, or fibrosis). Similar findings were obtained with furosemide. Substitution of electrolytes via the drinking water reduced these effects. However, substitution with NaCl and KCl in the food guaranteed full supplementation and fully prevented the alterations up to 4.8 mg muzolimine/kg orally. The “toxic” effects of high doses of potent diuretics are apparently the result of the otherwise desired electrolyte and volume loss. This condition must be taken into account when evaluating the toxicity of diuretics.

Renal effects of 1,4-dihydropyridines in animal models of hypertension and renal failure.

Renal effects of 1,4-dihydropyridine (DHP)-type calcium antagonists (nitrendipine and nisoldipine) were analyzed in diverse conditions, such as long-term antihypertensive treatment, acute saline-loading, and acute renal failure in rats. In spontaneously hypertensive rats (SHR), 60-week treatment with nitrendipine resulted in normotensive blood pressure values without increasing body weight, an indicator of salt-water retention, or increasing plasma renin activity and plasma aldosterone concentration compared with the untreated rats. After acute saline-loading of normotensive or hypertensive rats, administration of calcium antagonists nitrendipine and nisoldipine increased urinary volume and sodium excretion. This was in contrast to the effects observed with the vasodilator minoxidil, with which salt-water retention was shown. In acute renal failure induced by 60-min renal ischemia in uninephrectomized rats, administration of nisoldipine decreased mortality rate and improved kidney function. The increase in renal tissue calcium content and the decrease in ATP content associated with the renal failure was abolished by nisoldipine treatment. In conclusion, renal protective effects are present with DHP-type calcium antagonists; however, mechanisms in situations such as hypertension or acute renal failure might be different and deserve further analysis.

Effects of different antihypertensive drug classes on survival in animal models

Data on the influence of antihypertensive drug treatment on mortality of hypertensive rats are reviewed. Dihydropyridine calcium antagonists, verapamil, the angiotensin-converting enzyme (ACE) inhibitor captopril, and a triple combination of reserpine, hydralazine, and chlorothiazide normalized or markedly prolonged survival. Captopril was less effective in sodium chloride-induced, low-renin Dahl rat hypertension. Dihydralazine prolonged but did not nearly normalize survival. The K(+)-channel activator minoxidil was relatively ineffective. Data on diuretics or beta-blockers are insufficient or unavailable. Calcium antagonists nitrendipine and nimodipine and the ACE inhibitor captopril improved survival and prevented vascular lesions and calcinosis even at doses that failed to achieve normotension. All drugs that normalized survival also reduced heart weights. Minoxidil invariably increased heart weights and failed to improve survival. (Di)hydralazine assumed an intermediate position.

Influence of muzolimine on arterial wall elastin

Muzolimine, 3-amino-1-(3,4-dichloro-alpha-methylbenzyl)-2- pyrazolin -5-one, an antihypertensive and diuretic drug, accumulates in the arterial tissue of rats and dogs after oral administration. Two weeks after the administration of 3 mg [14C]muzolimine, the aorta of rats contained 60-300 times more 14C-radioactivity/weight unit than the skin or tail tendon. The 14C-radioactivity was exclusively bound to the isolated aortic elastin and corresponded to 0.04% of the applied muzolimine dose. Up to ca 250 ng bound muzolimine/mg elastin was found in the aorta of dogs treated with non-labelled muzolimine for 52 weeks. The elastin-bound [14C]muzolimine was not extractable by organic solvents or by weak acids or bases but was released in a soluble form by pancreatic elastase and extracted from the elastase digest by dichloromethane. In the dichloromethane extract muzolimine was detected by HPLC and HPTLC, and was identified by mass spectrometry. Muzolimine pretreatment of rats for 2 months did not influence the elastin content of arterial tissue or [3H]glycine incorporation into aortic elastin under organ culture conditions, but after labelling the elastin with [4,5-3H]lysine, the [3H]desmosine and [3H]-isodesmosine isolated from the elastin of muzolimine-pretreated rats and incorporated under organ culture conditions was lower than that of control animals. In addition, aortic elastin of rats pretreated for 2 months with 800 ppm muzolimine in the diet was more resistant to elastase degradation. This effect might give some implications for muzolimine in the therapy of cardiovascular disorders with impaired arterial elastin metabolism.

Protection Against Hypertensive Cardiovascular Damage by Dihydropyridine Calcium Antagonists

It is commonly acknowledged that calcium antagonists inhibit transmembrane calcium influx in arteriolar smooth muscle cells and thus reduce total peripheral resistance. This mechanism of action is supposed to be the prime basis of the antihypertensive action of these agents. However, despite their action as arteriolar vasodilators, therapeutically, calcium antagonists differ from vasodilators with other mechanisms of action (e.g., hydralazine or minoxidil) in having a marked natriuretic and diuretic action [6,8,17] and have been shown to preserve structure and function of heart, kidney, and mesenteric arterioles of hypertensive rats [2–4,10,13,18,24–26]. Therefore, tissue-protective effects of dihydropyridine calcium antagonists which might be unrelated to their hemodynamic actions, might add significantly to their long-term therapeutic efficacy in hypertension. We investigated the antihypertensive and tissue-protective effects of dihydropyridine calcium antagonists in long-term experiments: (1) nisoldipine in salt-loaded Dahl S and R rats in a preventive and therapeutic experiment, (2) nitrendipine in spontaneously hypertensive rats and Wistar-Kyoto rats in comparison to a classical vasodilator, and (3) nimodipine in stroke-prone spontaneously hypertensive rats in comparison to parathyroidectomy.

Experimental Hypertension and Therapeutic Progress: Vasodilation and Beyond

Inhibition of atrial natriuretic peptide (ANP) degradation by enkephalinase is known to increase endogenous ANP levels and to induce similar functional effects as administration of exogenous ANP. The influence of this principle on cardiac function in rats with hypertension-induced heart failure was investigated. We used 13-month-old male stroke-prone spontaneously hypertensive rats (SHR) with heart failure and hypertrophy. Groups of 12 rats were treated either with the enkephalinase inhibitor sinorphan (SIN; 31.5 mg/kg bj.d.), 50 ppm hydrochlorothiazide (HCTZ), in the feed or vehicle only for 14 days. SIN and HCTZ had similar acute natriuretic effects. Chronic treatment failed to affect natriuresis. SIN increased plasma and urinary cyclic guanosine monophosphate (cGMP). Unlike HCTZ, SIN reduced left ventricular enddiastolic pressure and heart weights. HCTZ failed to affect cardiac function or hypertrophy, but increased plasma renin activity. These reSults suggest that therapeutic use of ANP can improve cardiac function and structure in hypertension-induced cardiac failure. It was also shown that standard natriuretic treatment was ineffective.

Protective Effects of Calcium Antagonists on Hypertensive Diseases in Heart, Brain, and Kidney of Hypertensive Rats

The most evident result of calcium antagonist action in hypertension is the lowering of blood pressure. However, the protective effects of this type of agents on tissue structure and function may be largely independent of their vasorelaxant action. The natriuretic effect of dihydropyridine calcium antagonists is found in their antihypertensive dose range and was demonstrated to prevent volume expansion in (1-kidney, 1-clip)-hypertension. The functional recovery of the post-ischemic kidney was improved by treatment with nifedipine. With the help of biopsies of small mesenteric arteries of salt-loaded Dahl-S-rats with preexistent hypertension before arid after 6 weeks of nifedipine, the formation of new arterial endothelium and internal elastic lamina was observed. Morbidity, mortality, and incidence of organ lesions were largely reduced or prevented by the treatment of adult stroke-prone SHR with blood-pressure-neutral doses of nitrendipine or nimodipine. Nimodipine was shown largely to normalize brain and kidney calcium concentrations without altering serum ionized calcium concentration or blood pressure. It is concluded that the direct protective effects of calcium antagonists on cell structure and function might be therapeutically more important than their depressor effect.

Nifedipine corrects the blunted renal response to saline loading in hypertension-prone SBH rats.

The elimination of an acute oral saline load is markedly blunted in adult Sabra hypertension-prone (SBH) rats compared with hypertension-resistant Sabra normotensive (SBN) rats. Within 2 h, urinary output and the excretion of sodium and potassium are significantly reduced, while urine osmolality is markedly elevated in SBH rats. The long-term administration of nifedipine, 20–30 mg/kg body weight enhanced the diuretic and natriuretic response to saline loading in members of both strains. The effect was significantly more pronounced in SBH, especially in adult animals where the diuretic and natriuretic response averaged 150 and 130% of control, while in SBN the enhanced response was 50 and 20%, respectively. As a result of the disparate effect of nifedipine in the two strains, the blunted response of SBH was abolished. The mechanism of the preferential response to nifedipine of SBH rats remains to be determined.