Interference of calcium entry blockade in vivo with pressor responses to alpha-adrenergic stimulation: effects of two unrelated blockers on responses to both exogenous and endogenously released norepinephrine

Clevidipine resistance in a patient taking aripiprazole and methylphenidate

Various factors may be responsible for blood pressure alterations during perioperative care. When these physiologic alterations require treatment, several therapeutic options are available. Clevidipine is an ultrashort-acting, intravenous L-type calcium channel antagonist of the dihydropyridine class. Anecdotal experience has demonstrated its efficacy in various clinical scenarios in the pediatric population. We report apparent resistance to the vasodilatory effects of clevidipine in a 13-year-old girl who presented for anesthetic care during posterior spinal fusion for neuromuscular scoliosis whose chronic medication regimen included aripiprazole and methylphenidate for the treatment of depression and attention-deficit/hyperactivity disorder. We discuss the potential interaction of aripiprazole and methylphenidate with the calcium channel antagonists and cellular mechanisms responsible for the resistance to the vasodilatory effects of clevidipine.

Calcium channel blockers, postural vasoconstriction and dependent oedema in essential hypertension

Treatment with calcium channel blocker (CCB)s, dihydropyridines and others, is frequently complicated by dependent oedema in the absence of sodium retention or cardiac failure, a bothersome side effect of unclear aetiology. The present paper reviews our own and other work dealing with the antagonism exerted by such drugs on postural vasoconstriction, a mechanism triggered by limb venous congestion during orthostasis and controlled through a local sympathetic axo-axonic reflex and increased myogenic tone in response to changes in transmural pressure. By stabilising capillary pressure, postural vasoconstriction counteracts fluid hyperfiltration consequent to gravitational stimuli, and consistent evidence shows attenuation of this response by L-type calcium channel blockers. Interference with the postural reflex control of skin blood flow may therefore contribute to dependent oedema, although cannot entirely explain its development. Attenuation of postural vasoconstriction may amplify the fluid hyperfiltration induced by CCBs through other mechanisms, such as imbalanced intracapillary pressure or enhanced vascular permeability, which are the main factors determining net fluid filtration into the interstitial compartment.

Pharmacokinetics and pharmacodynamics of a slow-release formulation of diltiazem after the administration of a single and repeated doses to healthy volunteers

Diltiazem is a calcium antagonist used in angina pectoris and hypertension. There is little information concerning the slow-release (SR) formulation in the literature. The pharmacokinetics of diltiazem SR (120 mg) have been assessed over a 36h period in healthy volunteers after single- (SD) and multiple-dose (MD) administrations. Cmax, AUC0-36, and AUC0-infinity were significantly increased at steady state compared to the extrapolated SD values, suggesting accumulation of the drug. Renal and cardiovascular parameters have also been assessed at intervals of 3-6h during baseline (B) and following single and multiple doses of diltiazem SR. Diuresis over a 24 h period was increased, but not significantly, by the administration of diltiazem SR i.e. 1782 ml (MD) and 1915 ml (SD), versus 1626 ml (B). Natriuresis and creatinine clearance were slightly decreased by diltiazem SR, compared to B values; this might be due to the relatively short period over which steady state was maintained (five days) and the effects of norepinephrine and angiotensine II on renal vasculature and the pharmacokinetics of diltiazem SR. No increase in the systolic blood pressure occurred after the administration of diltiazem SR; diastolic blood pressure and PR interval were decreased and increased respectively by diltiazem SR. These results do not appear to be clinically significant. Finally, no relation was found between the pharmacokinetics and pharmacodynamics of diltiazem. This may be attributed to the absence of clinically significant effects in healthy volunteers, the presence of active metabolites, the pharmacokinetics of the SR formulation and/or the accumulation of the drug at steady state.

Response of the rat mesenteric vasculature to chronic treatment with nitrendipine alone and in combination with atenolol: evidence of a significant drug interaction

1. Nitrendipine 3 mg kg-1 was administered alone and in combination with atenolol 50 mg mg-1 day for 21 days to male normotensive Wistar rats and to spontaneously hypertensive Japanese Okamoto rats. Blood pressure was monitored daily. 2. Systolic blood pressure was decreased in normotensive Wistars and SHRs by nitrendipine alone and in combination. In both cases the decrease was greater in the hypertensive animal. There was no evidence of the combination having an additive hypotensive effect. 3. Following treatment, the response to exogenous noradrenaline (NA) and periarterial nerve stimulation (PNS) was measured in the in situ blood perfused mesentery. 4. Treatment with nitrendipine and the combination reduced the response to exogenous noradrenaline; with both, the reduction was greater in the hypertensive animal. The combination produced a larger reduction in response than nitrendipine alone. 5. Treatment with nitrendipine alone reduced the response to PNS in both normotensive and hypertensive animals, although this effect was greater in the SHR. 6. Combination treatment failed to change the response to electrical stimulation in the SHR, while in the normotensive rat it resulted in a large increase in response to higher frequency (16 and 35 Hz) stimulation. 7. As nitrendipine given alone reduced the response to PNS, and as we have previously shown a similar effect with atenolol given alone (Draper, Kingsbury, Redfern & Todd, 1992), the effect of the combination of nitrendipine and atenolol on responses to PNS is apparently influenced by some interaction between the two drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneous interference of nicardipine, verapamil, and diltiazem with forearm arteriolar responsiveness to adrenergic stimulation in hypertensive patients

The interference of intraarterial nicardipine, verapamil, and diltiazem with the forearm vascular response to graded exogenous norepinephrine was evaluated in hypertensive patients. Nicardipine antagonized the vasoconstrictor effect of norepinephrine in a dose-dependent manner, whereas verapamil was ineffective, suggesting that functional alpha-adrenergic antagonism may participate in the vasodilatory and possibly the antihypertensive effect. Nicardipine also blunted the vasoconstriction to lower-body negative pressure and the action of angiotensin II administered intraarterially. Despite a comparable increase in basal forearm flow, verapamil was less potent than nicardipine in inhibiting vasoconstriction after both stimuli. Therefore nicardipine suppressed preferentially regional vascular reactivity, probably by blockade of the influx of extracellular calcium in response to receptor activation, because both alpha-adrenergic and angiotensin II receptor-mediated vasoconstrictor responses were attenuated. At variance with both nicardipine and verapamil was potentiation of the responses to norepinephrine after the administration of diltiazem. Because intraarterial propranolol abolished that potentiating action and the local vasodilatation to isoproterenol was clearly reduced, diltiazem probably interfered also with beta-adrenergic receptor-mediated vasorelaxing mechanisms in human forearm arterioles. The data further stress the heterogeneity of calcium channel blockers in humans.

Differential effects of calcium antagonists and Bay K 8644 on contractile responses to exogenous noradrenaline and adrenergic nerve stimulation in the rabbit ear artery

1. The effects of three calcium antagonists (nifedipine, verapamil, diltiazem) and the calcium agonist Bay K 8644 were compared on contractile responses of similar amplitude elicited by noradrenaline (NA) and electrical nerve stimulation (ENS) in the rabbit isolated ear artery. 2. Contractions induced by both NA (3 x 10(-7) M) and ENS (10 Hz, 10s) were almost exclusively mediated by alpha 1-adrenoceptors, since 10(-7) M prazosin abolished (NA) or almost abolished (ENS) the responses, and prazosin was more than three orders of magnitude more potent than rauwolscine on both types of response. 3. ENS-induced contractions were considerably less inhibited by nifedipine, verapamil and diltiazem than were those elicited by NA. Bay K 8644 enhanced responses to NA more than those to ENS. 4. The inhibitory effect of nifedipine and Ca2+ deprivation on NA-induced contractions decreased with increasing NA concentration. Reduction of the NA response by prazosin or phenoxybenzamine increased the nifedipine inhibition. 5. Reduction of the ENS-induced contractions by prazosin or phenoxybenzamine, or by use of a lower stimulation frequency did not increase the inhibitory effect of nifedipine. 6. In conclusion, the differential effects of the calcium antagonists on NA- and ENS-induced contractions were not related to differences in alpha-adrenoceptor subtype (alpha 1/alpha 2), receptor reserve or response amplitude, but may rather reflect temporal and spatial differences in alpha-adrenoceptor activation between the responses.

Sustained-release nifedipine (nifedipine-L) suppresses plasma thromboxane B2 and 6-keto prostaglandin F1 alpha in both young male smokers and nonsmokers

Sustained-release nifedipine (nifedipine-L) (40 mg twice a day) was administered orally to healthy young adult male smokers and nonsmokers for 10 days, and its effects on platelet aggregation, beta-thromboglobulin and platelet factor 4 levels, and plasma thromboxane B2 (TxB2) and 6-ketoprostaglandin F1 alpha (6-Keto-PGF1 alpha) concentrations were studied. The plasma nifedipine-L concentration in smokers (46.0 +/- 7.4 ng/ml) was significantly lower than that in nonsmokers (88.2 +/- 1.2 ng/ml). Nifedipine-L did not affect platelet aggregation induced by adenosine diphosphate, collagen, or epinephrine in either smokers or nonsmokers. The plasma beta-thromboglobulin level on the tenth day of nifedipine-L administration in nonsmokers was lower than that in smokers, but there were no significant differences either with or without nifedipine-L or between nonsmokers and smokers. Nifedipine-L had no effect on the concentration of platelet factor 4 in either smokers or nonsmokers. On the other hand, nifedipine-L significantly decreased the plasma TxB2 and 6-keto-PGF1 alpha concentrations in both smokers and nonsmokers. Thus we concluded that nifedipine-L suppressed the production of plasma TxB2 from platelets and also subsequently suppressed the production of 6-keto-PGF1 alpha and that this action was not affected by cigarette smoking.

Verapamil. An updated review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in hypertension

Although verapamil is a well-established treatment for angina, cardiac arrhythmias and cardiomyopathies, this review reflects current interest in calcium antagonists as anti-hypertensive agents by focusing on the role of verapamil in hypertension. Verapamil is a phenylalkylamine derivative which antagonises calcium influx through the slow channels of vascular smooth muscle and cardiac cell membranes. By reducing intracellular free calcium concentrations, verapamil causes coronary and peripheral vasodilation and depresses myocardial contractility and electrical activity in the atrioventricular and sinoatrial nodes. Verapamil is well suited for the management of essential hypertension since it produces generalised systemic vasodilation resulting in a marked reduction in systemic vascular resistance and, consequently, blood pressure. Evidence from clinical studies supports the role of oral verapamil as an effective and well-tolerated first-line treatment for the management of patients with mild to moderate essential hypertension. Clinical studies have shown that verapamil is more effective the higher the pretreatment blood pressure and some authors have found a more pronounced antihypertensive effect in older patients or in patients with low plasma renin activity. Sustained release verapamil formulations are available for oral administration which, as a single daily dose, are as effective in lowering blood pressure over 24 hours as equivalent doses of conventional verapamil formulations given 3 times daily. As a first-line antihypertensive agent, oral verapamil is equivalent to several other calcium antagonists, beta-blockers, diuretics, angiotensin-converting enzyme (ACE) inhibitors and other vasodilators, and is not associated with many of the common adverse effects of these treatments. Verapamil may be preferred as an alternative first-line antihypertensive treatment to diuretics in elderly patients because it has similar efficacy in these patients without causing the adverse effects commonly linked with diuretic treatment. Furthermore, because verapamil does not cause bronchoconstriction, it may be used in preference to beta-blockers in patients with asthma or chronic obstructive airway disease. Reflex tachycardia, orthostatic hypotension or development of tolerance is not evident following verapamil administration. As a second- or third-line treatment for patients refractory to established antihypertensive regimens, verapamil produces marked blood pressure reductions when combined with diuretics and/or ACE inhibitors, beta-blockers and vasodilators such as prazosin.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacology of acute effort angina

From the pharmacologic point of view, each of the major types of antianginal agents--calcium antagonists, beta-blockers, and nitrates--seem to act at least in part by an improvement of the myocardial blood supply. The recently elucidated mechanism of action of nitrates, acting on a common pathway with the endothelium-derived relaxation factor (EDRF), suggests an important role for guanylate cyclase and cyclic GMP in maintaining coronary artery patency in patients with coronary atheroma. The efficacy of calcium antagonists, even in effort-induced angina, is in accord with a current hypothesis that physical exercise in the presence of coronary stenosis can cause relative coronary vasoconstriction, or at the least, failure of full dilation. Therefore, calcium antagonists all act, at least in part, on the "supply" side of the supply-demand equation. Beta-adrenergic blockers appear to have as their major mode of action a reduction of heart rate, which not only reduces the oxygen demand but, through an anti-ischemic effect, also appears to improve the endocardial blood supply (in relation to the heart rate). Thus beta-blockade indirectly enhances the supply side of the equation. The intriguing situation arises whereby all three major types of antianginal compounds may also act by a common mechanism of anginal relief, namely, improvement in the coronary blood supply, in addition to the diverse mechanisms specific to each type of compound. That conclusion does not mean the the "demand" side of the equation can be ignored. Rather, the critical importance of a reduced myocardial blood supply in the production of anginal syndromes is highlighted.

Effects of preoperative calcium channel and beta-adrenergic blockade on the vascular response to adrenergic agonists during cardiopulmonary bypass

The present report compares the effects of isoproterenol (ISO), norepinephrine (NE), and phenylephrine (PH) on the mean arterial pressure (MAP) and reservoir volume (RV) during cardiopulmonary (CPB) in 16 patients undergoing coronary artery bypass grafting (CABG) who were treated preoperatively with oral nifedipine (10 to 40 mg, three times a day) and propranolol (40 to 60 mg, three times a day). The changes of RV and MAP were used as indices of the changes in venous capacitance and arterial resistance, respectively, produced by the adrenergic agonists. ISO, a beta-adrenergic agonist, decreased both MAP and RV. NE, which activates both alpha 1- and alpha 2- adrenoceptors, increased both MAP and RV, while PH, a selective alpha 1-adrenergic agonist, increased only MAP with no significant change in RV. The changes are qualitatively similar to those previously achieved in patients undergoing valve replacement who did not receive any blocker preoperatively. However, the decrease of MAP by ISO and its increase by NE were significantly less in the CABG group. It is concluded that preoperative preparation of patients undergoing CABG with beta-adrenergic blockers and calcium channel blockers can modify the effect of ISO and NE on the peripheral resistance, but they have no significant effect on the action of PH at the doses selected in this study.

Nifedipine and alpha-adrenoceptor function in human finger skin vessels

Experiments were designed to study the effect of the calcium entry blocker nifedipine on contractions evoked by alpha-adrenoceptor activation in human finger skin vessels and to show whether nifedipine could prevent the vasoconstriction induced by local cooling. Vasoconstriction of finger skin vessels was evoked with phenylephrine (a preferential alpha 1-adrenoceptor agonist), B-HT933 (a preferential alpha 2-adrenoceptor agonist) and by local cooling. The alpha-adrenoceptor agonists were administered into the skin by iotophoresis and local cooling was produced with a Peltier element. Nifedipine significantly attenuated the vasoconstrictor responses both to alpha 1- and alpha 2-adrenoceptor stimulation, the effect on alpha 2-mediated responses being more pronounced. It had no effect on the vasoconstriction induced by local cooling of finger skin.

Calcium antagonists and their potential for antihypertensive therapy

Calcium antagonists are potent arterial vasodilators devoid of relevant chronic sympathetic reflex activation and sodium and volume retention. This favorable hemodynamic profile of action renders them suitable for monotherapy of hypertension where they act to reduce an enhanced, calcium-influx-dependent vasoconstrictor mechanism which may be brought about by altered smooth muscle cation handling and increased intracellular free calcium concentrations. Clinical studies have proved their efficacy, safety, and good tolerability alone or in combination with other drugs in uncomplicated hypertension where they are particularly effective in older and low-renin and possibly black patients. These properties and their efficacy in the treatment of severe and accelerated hypertension or hypertensive emergencies make them a valuable addition to already available drug therapy.

Role of calcium channel blockers in experimental exercise-induced ischemia

Calcium channel blockers, which induce vasodilation by relaxing vascular smooth muscle cells, have proven effective in the treatment of angina pectoris. To study mechanisms of calcium blockade in ischemic heart disease, conscious chronically instrumented dogs with a single coronary artery ameroid constrictor were studied during steady-state treadmill runs which induced regional myocardial ischemia. During exercise-induced ischemia, regional systolic wall thickening and subendocardial blood flow were both significantly reduced in the ischemic zone. Calcium channel blockade with verapamil, diltiazem, or nifedipine enhanced regional systolic wall thickening. Regional subendocardial blood flow in the ischemic region, measured during diltiazem and nifedipine experiments, improved during exercise. Reduced coronary artery resistance in the native vessels and/or recruitment of collaterals appears to largely explain the increased total myocardial blood supply in the jeopardized area and the increased function. However, after diltiazem, reduced exercise heart rates as well as reduced left ventricular end-diastolic pressure also contributed to the improvement in the oxygen-supply imbalance in the ischemic myocardium. These data provide a basis for understanding the efficacy of calcium channel blocker treatment in patients with coronary artery disease.

Long-term renal effects of diltiazem in essential hypertension

Eighteen hypertensive patients underwent short-term (8 weeks) and long-term (6 months) assessment of renal function and body fluid composition following diltiazem monotherapy (240 to 480 mg/day). Diltiazem monotherapy effectively lowered blood pressure in 60% of patients at 8 weeks. In 12 of the 18 patients continued on diltiazem monotherapy for 6 months, good control of blood pressure was sustained. Glomerular filtration rate, effective renal plasma flow, filtration fraction, and renal vascular resistance were unchanged throughout the protocol period. In individuals with pretreatment glomerular filtration rates less than or equal to 80 ml/min/1.73m2, diltiazem monotherapy showed both short-term and long-term improvement in glomerular filtration rate (62%) and effective renal plasma flow (34%). Filtration fraction was unchanged, suggesting that the changes in glomerular filtration rate might be related to the attenuated intrarenal effects of angiotensin II and/or norepinephrine. No long-term effect was seen on salt and water excretion or body fluid composition.

Acetate relaxation of isolated vascular smooth muscle

The vasorelaxant effects of acetate in arginine vasopressin (AVP)-contracted rat tail artery strips were examined in order to study mechanism of action. Dose-dependent relaxation by acetate was found in the clinically important range of 4 to 16 mM. Relaxation was not due to complexing of ionized calcium, persisted after mechanical removal of the endothelium, and was not altered by pretreatment with indomethacin. Although acetate also inhibited contraction by alpha-1 and alpha-2 agonists, the relaxant effect was not altered by destruction of sympathetic nerve terminals using 6-hydroxydopamine. The degree of relaxation in this model by various anions correlated with their lyotropic properties; however, the vasorelaxant effect of acetate exceeded that which would be expected on the basis of its position in the lyotropic series. The vasorelaxant effect of acetate was shared by other short-chain fatty acids that can be conjugated with coenzyme A (CoA), such as propionate and malonate. In contrast, a much lesser or absent relaxant effect was found with nonfatty-acid precursors of acetyl CoA, such as pyruvate, lactate, and alanine. The vasorelaxant effect of acetate was abolished by pretreatment with DIDS, an inhibitor of organic anion uptake, suggesting that cellular uptake of acetate is essential to its vasorelaxant action. The results suggest that the relaxant effect of acetate in vascular smooth muscle is non-specific, is not mediated by prostaglandins, does not depend upon the presence of either endothelium or the sympathetic nervous system, and may be due to metabolism of acetate to acetyl CoA with attendant conversion of ATP to AMP.

Hemodilution affects the pressor response to norepinephrine

A hindlimb preparation of 19 male rats was used to determine the responses to norepinephrine (NE) at different levels of hemodilution at a constant flow, at increased flow and maintained pressure, and after reversal of hemodilution. An initial series demonstrated that the preparation remained stable with unchanged blood pressure, plasma electrolytes, and blood gases; and had reproducible responses to NE throughout the experiment. Two levels of isovolumic hemodilution were achieved by blood/hetastarch exchanges equivalent to 1.5% and 3.0% body weight, which reduced hematocrit to 30.8 +/- 2% and 18.7 +/- 1.5%, respectively. Hemodilution reduced the perfusion pressure from 73.6 mmHg to 58.6 and 55.7 mmHg following 1.5% and 3.0% hemodilution, respectively. A dose-response relationship to NE (0.1, 0.3, 1.0 micrograms/kg) was obtained. The responses to NE were markedly reduced following both 1.5% and 3.0% hemodilution. Retransfusion of packed red blood cells increased the hematocrit from 21.1 +/- 2.2% during hemodilution to 41.2 +/- 2.1% (P less than .01). Concomitantly, perfusion pressure increased from 46.2 +/- 3.6 to 68.3 +/- 6.5 mmHg (P less than .001), close to prehemodilution levels (65.3 +/- 5.3 mmHg). Restoration of hematocrit levels restored responses to NE. In a third series, hemodilution was continued, but the blood flow was increased from 4.0 to 8.3 +/- 0.9 mL/min, and perfusion pressure returned to baseline values. This also restored the pressor responsiveness to NE to near normal.

Short- and long-term effects of calcium entry blockers on the kidney

The renal effects of the calcium entry-blocking drugs diltiazem, nifedipine, nitrendipine, nicardipine and verapamil are reviewed. Although nifedipine may acutely increase plasma renin activity, most of the calcium entry blockers have no sustained effect on any of the components of the renin-angiotensin-aldosterone system. Although all of the calcium entry blockers effectively lower blood pressure, none adversely affects renal function: Glomerular filtration rate and effective renal plasma flow are maintained. Diltiazem may increase glomerular filtration rate via attenuation of the intrarenal effects of angiotensin II or norepinephrine. Although all of the calcium entry blockers acutely increase salt and water excretion, most of the calcium entry blockers have no clinically sustained effect on salt and water excretion; serum electrolytes, urinary sodium and potassium excretion, body fluid composition and body weight are usually unchanged. Calcium entry blockers can be expected to assume a prominent role in the treatment of hypertension because of their ability to lower blood pressure while preserving renal perfusion and function.

Diltiazem as monotherapy for systemic hypertension: a multicenter, randomized, placebo-controlled trial

A multicenter, randomized, placebo-controlled, parallel group study of diltiazem in essential hypertension was carried out in 77 patients (40 diltiazem, 37 placebo) with stable supine diastolic blood pressure (BP) between 95 and 110 mm Hg. Patients were withdrawn from previous antihypertensive therapy for at least 4 weeks, titrated to the optimal dose, and followed for a total of 12 weeks during therapy. A diltiazem dose of 360 mg/day was required in 85% of the patients. Average BP in all positions was significantly (p less than 0.0001) reduced by diltiazem compared with placebo. With diltiazem, average supine BP fell from 156/100 mm Hg at baseline to 141/87 at end titration and 145/90 mm Hg at week 12, whereas average standing BP fell from 152/101 mm Hg to 136/90 and 143/91 mm Hg, respectively, at those times. There was no significant change in heart rate at week 12. Diltiazem tended to be more effective in older patients, but caused no increase in orthostatic BP drop. There were no statistically significant changes in BP in the placebo group. Two patients receiving placebo and 1 patient receiving diltiazem discontinued therapy as a result of adverse effects, and overall, side effects were only slightly more common with diltiazem treatment. Thus, diltiazem was effective and well tolerated single therapy for mild to moderate systemic hypertension and appears to compare favorably to most agents being used.

Effects of calcium entry blockers on renin-angiotensin-aldosterone system, renal function and hemodynamics, salt and water excretion and body fluid composition

The renal effects of the calcium entry-blocking drugs diltiazem, nifedipine, verapamil and nitrendipine are reviewed. Although nifedipine stimulates plasma renin activity on a short-term basis, none of the calcium entry blockers produces a clinically significant sustained effect on any of the components of the renin-angiotensin-aldosterone system. Although all of the calcium entry blockers effectively lower blood pressure, none adversely affects renal function; glomerular filtration rate and effective renal plasma flow are maintained. Diltiazem may increase glomerular filtration rate via attenuation of the intrarenal effects of angiotensin II or norepinephrine. Although diltiazem and nifedipine increase salt and water excretion on a short-term basis, none of the calcium entry blockers produces a clinically significant sustained effect on salt and water excretion; serum electrolytes, urinary sodium and potassium excretion, body fluid composition and body weight are unchanged. Thus, calcium entry blockers can be expected to assume a prominent role in the treatment of hypertension because of their ability to lower blood pressure while preserving renal perfusion and function.