Negative Inotropic Effect Of Nifedipine Research Articles

Developmental Changes in Excitation–Contraction Mechanisms of the Mouse Ventricular Myocardium as Revealed by Functional and Confocal Imaging Analyses

Developmental changes in excitation–contraction mechanisms were examined in the ventricular myocardium from fetal, neonatal, and 1-, 2-, and 4-week-old mice. In isolated tissue, the negative inotropic effect of nifedipine decreased, while that of ryanodine increased with age. Action potential duration decreased with age, especially during the late fetal period. In ventricular cardiomyocytes, fluorescence imaging revealed that the sarcoplasmic reticulum increases progressively during pre- and postnatal development. t-Tubules were absent in the fetus and neonate, were observed only in the subsarcolemmal region at 1 week after birth, and were present throughout the cytoplasm at 2 and 4 weeks after birth. The amplitude of Ca2+ transients, as well as its ryanodine-sensitive component, increased with age. In the neonate and 1-week-old mice, Ca2+ at the cell center showed slower rise than the subsarcolemmal region, but in 2- and 4-week-old mice, Ca2+ increased simultaneously across the entire width of the cell. These results suggest that in the mouse ventricular myocardium, the shortening of the action potential during the late fetal period and the development of t-tubule–sarcoplasmic reticulum coupling during the second postnatal week largely contribute to the developmental increase in the dependence of contraction on sarcoplasmic reticulum function.

Cross-Bridge-Dependent Change in Ca2+ Sensitivity is Involved in the Negative Inotropic Effect of Nifedipine in Aequorin-Injected Ferret Ventricular Muscles

We hypothesized that the negative inotropic effect of nifedipine (Nif) on cardiac ventricular muscle is partly due to the cross-bridge-dependent decrease of Ca2+ sensitivity of the myofilaments as well as the decrease in Ca2+ influx. We used aequorin-injected ferret papillary muscles and measured the slope of the extra-Ca(2+)-tension relation which expresses the change in the Ca2+ sensitivity through the feedback from the cross-bridges. Twitch tension was decreased significantly by 0.5 micromol/L Nif accompanying a significant reduction of the Ca2+ transient peak. When Nif (0.2-0.5 micromol/L) was added to the solution with 8 mmol/L Ca2+, the slope of the extra-Ca2+-tension relation became steeper in a concentration-dependent manner, which was similar to the change in the slope when the concentration of Ca2+ was decreased from 8 to 1 mmol/L in the absence of Nif. BAY-K 8644 (0.3 micromol/L), a dihydropyridine receptor agonist, showed the opposite effect on the slope of the extra-Ca2+-tension relation to that observed in Nif. However, 2,3-butanedione monoxime (3 mmol/L), an inhibitor of the active cross-bridges, antagonized the effect of BAY-K 8644. Nif exerts its negative inotropic effect on cardiac muscle by suppressing Ca2+ binding to troponin C via the inhibition of the L-type Ca2+ channel, and by the cross-bridge-dependent decrease in the Ca2+ sensitivity, as in low extracellular Ca2+ concentration.

Negative Inotropic Effect of Nifedipine in the Immature Rabbit Heart Is Due to Shortening of the Action Potential

Contractions in neonatal rabbit ventricular myocytes seem to depend predominantly on Ca2+ influx through the Na+-Ca2+ exchanger (NCX). Unexpectedly, neonates are sensitive to the negative inotropic effect of L-type Ca2+ channel blockers. L-type Ca2+ channel blockers depress contractile function indirectly in neonatal myocytes by shortening the action potential duration (APD), thereby decreasing the influx of activator Ca2+ through the NCX. Freshly isolated ventricular myocytes from adult and neonatal (1-5 d) rabbits were electrically stimulated (0.5 Hz; 35 degrees C) while action potential (AP) and Ca2+ transients (Indo-1) were recorded in the absence and presence of nifedipine (10 microM). In separate experiments, cells were voltage-clamped with a constant AP waveform (APD90=170 ms) to determine the effect of nifedipine on Ca2+ transients independent from effects on the AP. Voltage-clamp experiments confirmed that nifedipine blocks L-type Ca2+ current in neonatal myocytes. Nifedipine markedly reduced Ca2+ transient amplitude and APD in both adults (transient=20 +/- 7%; APD90=31 +/- 4% of control) and neonates (transient=38 +/- 10%; APD90=57 +/- 6% of control). When the AP was held constant by voltage clamping, nifedipine significantly reduced the amplitude of Ca2+ transients in adults (27 +/- 9% of control) but had no effect on Ca2+ transient amplitude in neonatal myocytes. These results are consistent with the concept that immature ventricular myocytes are less reliant on L-type calcium channels and are more dependent on NCX for contraction. The negative inotropic effect of L-type Ca2+ channel blockers in neonates is attributable to shortening of the AP.

GLUCAGON REVERSES THE EFFECT OF NIFEDIPINE ON CONTRACTIONS IN NEONATAL VENTRICULAR MYOCYTES. † 152

Glucagon has been shown to reverse the negative inotropic effects of Ca2+ channel blockers in adult dogs, rats, mice, and humans. The mechanism for this action of glucagon remains unclear, but one hypothesis proposes it is related to an increase in circulating cAMP, resulting in increased Ca2+ release from the sarcotubular system. To test this hypothesis and to determine if glucagon reverses the effect of Ca2+ channel blockers on contractions in immature cells, we studied the effects of glucagon on neonatal rabbit ventricular myocytes. Previous studies show sarcotubular development (and its contribution to contractions) is diminished in these cells. Cell shortening was measured in field-stimulated ventricular cells from neonatal (1-4 day old) New Zealand White rabbits using a video edge detector. The cells were bathed at 24°C., initially in normal Tyrodes solution containing (in mM) NaCl 138, Dextrose 11.0, MgCl2 1.0, KCl 4.4, CaCl2 2.7 and HEPES 12.0 (pH adjusted to 7.4 with NaOH). Maximum inhibition of contractions was achieved in the neonatal cells using 20 μM nifedipine. Contractions after nifedipine were 6.9±2.0%(mean±SEM) of baseline, but returned to 71.4±8.3% of baseline within 2 minutes of infusion with Tyrodes containing 1 μM glucagon (n=9). In contrast, contractions in cells treated with nifedipine and perfused for 2 minutes with Tyrodes alone (n=8) remained small compared to contractions in cells glucagon-treated (12.8±3.7% of baseline, P≤0.001), without significant increase from measurements after nifedipine. In conclusion, glucagon appears to affect mechanisms other than cAMP-induced Ca2+ release from the sarcotubular system. Since glucagon reverses the negative inotropic effect of nifedipine in neonatal myocytes, it may be useful clinically in infants requiring therapy with Ca2+ channel blockers.

Antiischemic Effects of Nifedipine in Isolated Working Heart Preparations of Healthy, Diabetic, and Hypertensive Rats

We evaluated the antiischemic effects of nifedipine in isolated working rat hearts from age-matched normotensive Wistar-Kyoto rats (WKY), diabetic WKY, spontaneously hypertensive rats (SHR), and diabetic SHR. Diabetes was induced by streptozotocin. First, we constructed concentration-response curves for the negative inotropic effect of nifedipine in every group. After 15 min of pretreatment with nifedipine (EC60), low-flow ischemia (30 min) was induced by reducing the afterload from 51.5 to 11.0 mm Hg and nifedipine was infused simultaneously. The six measured parameters were left ventricular pressure (LVP), maximum rate of pressure increase (+dP/dtmax), maximum rate of pressure decrease (-dP/dtmax), aortic output (AO), coronary flow (CF), and cardiac output (CO), determined after 15-min equilibration in the working heart mode and at the end of the experiment. From these data, the recovery percentages were calculated. There were no significant differences in sensitivity to nifedipine (as measured by the EC50 concentration) between the four groups with respect to LVP, +dP/dtmax, -dP/dtmax, CF, and CO. However, hearts from SHR were less sensitive to nifedipine than those from diabetic SHR and nondiabetic WKY with regard to AO. In isolated hearts from nondiabetic WKY and SHR, there were no significant differences between vehicle-treated organs and nifedipine-treated preparations. In hearts from diabetic WKY and diabetic SHR, however, the nifedipine-treated group (LVP 87.1 +/- 3.3 and 60.5 +/- 12.1%, respectively) recovered significantly (p < 0.05) better from ischemia as compared with the control group (LVP 35.7 +/- 14.7 and 10.7 +/- 9.8%, respectively) (n = 6 for each group).(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac inotropic responses to calcium and forskolin are not altered by prolonged isoproterenol infusion

Effects of prolonged isoproterenol infusion upon the density of cardiac calcium channels, calcium-mediated contractile responses, and the ability of forskolin to enhance tension development and cyclic AMP accumulation were studied in ventricular muscle preparations from Sprague-Dawley rats. Isoproterenol infusion (400 μg/kg per h s.c., 4 days) significantly decreased calcium channel density (B max) in cardiac microsomal membranes as quantified by a 32% decrease in specific [ 3H]nitrendipine binding sites; binding affinity (K D) was unchanged. A 57% decrease of β-adrenoceptors confirmed homologous down regulation. To examine functional effects of decreased [ 3H]nitrendipine binding sites, responses to calcium, BAY K8644 and nifedipine were determined in isolated right ventricular strips. Significant decreases in basal developed tension were observed in muscles from isoproterenal-infused rats. However, concentration-dependent increases in contractility in response to CaCl 2 or BAY K8644 were comparable, and the negative inotropic effect of nifedipine was unchanged. Whereas isoproterenol infusion was associated with significantly decreased basal cardiac cyclic AMP concentrations, exposure of ventricular strips from either vehicle- or isoproterenol-infused rats to 10 μM forskolin resulted in comparable increases in cyclic AMP and in developed tension. Cumulative, submaximal concentrations of forskolin also produced similar increases in contraclity with maximum responses in ventricular strips from vehicle-infused animals attained at 4.4 μM forskolin. Higher concentrations resulted in automaticity. By contrast, ventricle from isoproterenol-infused animals responded to 14.4 μM forskoklin with maximal increases in force of contraction.

Negative inotropic effect of intravenous nifedipine in coronary artery disease: Relation to plasma levels

The relative extent of the vasodilator versus direct negative inotropic effects of nifedipine was studied in 15 male patients with documented coronary artery disease and normal left ventricular function. At the time of diagnostic cardiac catheterization, three groups of five patients received dose of 1, 2, and 3 mg intravenous nifedipine at a rate of 0.33 mg/min. Hemodynamic measurements and blood collections were made before, during, and every 5 minutes for 30 minutes after infusion of nifedipine. Heart rate increased and mean arterial pressure decreased significantly after the 2 and 3 mg doses of nifedipine. Systemic vascular resistance was significantly decreased and cardiac index increased after all doses of nifedipine. Maximal left ventricular dp/dt (dp/dtmax) was significantly decreased after the 3 mg infusion. The reduction in dp/dtmax was most consistent with a reduction in left ventricular contractility as opposed to changes in loading conditions. Plasma concentrations of nifedipine were significantly correlated with bidirectional changes in dp/dtmax (r = 0.86). Nifedipine concentrations below 28.2 ng/ml were associated with a rise in dp/dtmax, whereas concentrations above that level were associated with a reduction in dp/dtmax. These data indicate that intravenous nifedipine produces dose- and concentration-dependent depression of myocardial contractility in patients with coronary artery disease. Nifedipine concentrations associated with negative inotropic effects are readily achievable with common oral and sublingual doses.

Effects of hypoxia, elevated K+ and acidosis on the potency of verapamil, diltiazem and nifedipine in the guinea-pig isolated papillary muscle.

1 The effects of the structurally diverse calcium channel blockers verapamil, nifedipine and diltiazem were investigated on the force of contraction of guinea-pig, electrically stimulated papillary muscles in vitro. 2 Calcium channel blocking potency was assessed either as a direct negative inotropic effect or as the ability of the drugs to antagonise the positive inotropic effects of added calcium (Ca2+). By either method, the rank order of potency was found to be nifedipine greater than verapamil greater than diltiazem. 3 Various factors which mimic some of the consequences of acute ischaemia in vivo, namely low pH, hypoxia and elevated K+, in combination, but not singly, enhanced the negative inotropic potency of verapamil and to lesser extent that of diltiazem, but not that of nifedipine. 4 Whilst the various interventions, especially in combination, produced a profound negative inotropic effect themselves, this was not responsible per se for the potentiation of the negative inotropic effects of verapamil and diltiazem, since the negative inotropic effects of nifedipine and dinitrophenol were not potentiated under the 'ischaemic' conditions. 5 By use of antagonism of the positive inotropic action of exogenous Ca2+ as an alternative measure of potency, the differential influence of 'ischaemia' on calcium channel blocker potency was confirmed. The effect of verapamil was potentiated some 9 fold, that of diltiazem about 2 fold, and that of nifedipine was unchanged. 6 The differential effect of 'ischaemia' on the potencies of the calcium channel blockers was unexpected. Verapamil (but not nifedipine) is thought to bind to the calcium channel at a specific site not easily accessible from the extracellular space. 'Ischaemic' conditions may cause membranal perturbations which allow verapamil easier access to its binding site thus increasing its negative inotropic potency.

Interaction between isoproterenol and dihydropyridines in heart cells

The negative inotropic potencies (IC 50) of the calcium channel blockers nicardipine (12 ± 2 nM) and nifedipine (9 ± 2 nM) measured in cultured chick embryo ventricular cells were not significantly different. Exposure of the cells to isoproterenol potentiated the negative inotropic effect of low concentrations of nicardipine (10 −9-10 −8 M) and decreased the effect of higher concentrations (10 −7 M). In contrast, isoproterenol shifted the entire dose-response curve for nifedipine to the right. These results provide evidence that activation of β-adrenoceptors modulates the negative inotropic effect of nifedipine and nicardipine differently.

Effects of Bay k 8644 on the Coronary Vascular Selectivity of the Dihydropyridine Ca Antagonists in the Canine Isolated, Blood-Perfused Papillary Muscle Preparation

Antagonism by Bay k 8644 of the coronary vasodilator and negative inotropic effects of Ca antagonists was simultaneously investigated in the canine isolated, blood-perfused papillary muscle electrically driven at 2 Hz. Drugs were given intraarterially. Continuously infused intraarterially, Bay k 8644 inhibited more predominantly the coronary vasodilator effects of the dihydropyridine Ca antagonists (nifedipine, nitrendipine, and nicardipine) than those of diltiazem and verapamil, or the negative inotropic effects of nifedipine and nitrendipine. The negative inotropic effects of nicardipine, diltiazem, and verapamil were not significantly affected by Bay k 8644. The dose ratio for producing a 50% decrease in developed tension of the papillary muscle over the dose for producing a 50% increase in coronary blood flow (which reflects vascular selectivity) were 41 for nitrendipine, 25 for nifedipine, 21 for nicardipine, 9 for diltiazem, and 7 for verapamil. During infusion by Bay k 8644, the ratio was small (2 to 6) and almost similar among all five Ca antagonists. These results indicate that Bay k 8644 antagonizes more effectively the coronary vasodilating effects of the dihydropyridine Ca antagonists, suggesting that there is a specific type of Ca channel on coronary vascular smooth muscles, through which the dihydropyridine Ca antagonists act.

Differential antagonism by Bay k 8644, a dihydropyridine calcium agonist, of the negative inotropic effects of nifedipine, verapamil, diltiazem and manganese ions in canine ventricular muscle.

Antagonism between either the dihydropyridine calcium agonist, Bay k 8644, or high external Ca2+ and the calcium antagonists, nifedipine, verapamil and diltiazem, and Mn2+ was investigated in canine isolated ventricular trabeculae. Bay k 8644 (10(-7)-10(-5)M) produced a slowly developing increase in developed tension which reached a maximum at 10(-6)M. A small decrease in the positive inotropic effect of Bay k 8644 at 10(-5)M was probably due to the negative inotropic effect of the solvent, 0.5% ethanol. Bay k 8644 (10(-7)-10(-5)M) produced a rightward parallel shift of the concentration-response curves for the negative inotropic effects of nifedipine (10(-8)-10(-5)M) and verapamil (10(-7)-3 X 10(-5)M). The slopes of the Schild plots were -0.92 for nifedipine (pA2 value = 6.58) and -0.48 for verapamil. Bay k 8644 (10(-6) and 10(-5)M) produced only a slight rightward shift of the concentration-response curves for the negative inotropic effect of diltiazem (10(-7)-3 X 10(-5)M) and did not affect the negative inotropic effect of Mn2+ (10(-4)-10(-2)M). Addition of 2.5 X 10(-3)M Ca2+ (5.05 X 10(-3)M Ca2+) to the medium produced a greater maximum positive inotropic effect than Bay k 8644. The concentration-response curves for the negative inotropic effects of nifedipine, verapamil and diltiazem obtained under these conditions were not essentially different from those under control conditions (2.55 X 10(-3)M Ca2+). 6 These results indicate that Bay k 8644, while producing a positive inotropic effect, antagonizes the negative inotropic effect of nifedipine by competing with the latter for the same site closely associated with the calcium channel. In contrast, Bay k 8644 antagonizes the negative inotropic effects of verapamil and diltiazem by interfering allosterically with the binding of these calcium antagonists to their sites of action. Bay k 8644 does not antagonize the negative inotropic effect of Mn2+. No pharmacological antagonism was observed between the three organic calcium antagonists and high external Ca2+_

Temperature and frequency dependence of the negative inotropic effect of nifedipine

Temperature and frequency dependence of the negative inotropic effect of nifedipine

The effects of anti-anginal drugs on the contraction of the blood-perfused canine papillary muscle under ischemia or non-ischemic hypoxia.

The isolated canine papillary muscle (PM) was perfused with blood through the anterior septal artery at a constant pressure of 100 mmHg and was stimulated at 1.5 Hz. The perfusion pressure was decreased to 40 mmHg (ischemia, IS), while in nonischemic hypoxia (NIHY), PM was perfused with the coronary sinus blood (CSB) or with the deoxygenated venous blood (DVB) by a membrane oxygenator (SciMed). PO2 (mmHg), PCO2 (mmHg) and pH were 17±1, 31±1 and 7.37±0.01 in CSB and 21±2, 24±2 and 7.39±0.01 in DVB, respectively. Drugs were injected into the anterior septal artery. The blood flow to PM changed to about 38, 200 and 260 % of the control in IS, NIHY with DVB, and NIHY with CSB, respectively. The contractile force (CF), ±dT/dt and time to peak tension did not change in IS and in NIHY with DVB, but. decreased significantly in NIHY with CSB. The maximum responsive frequency without alternative response decreased in IS or NIHY. Negative inotropic effects (NIE) of verapamil (1-30 μg) tended to be potentiated in IS and in NIHY with DVB and was significantly potentiated in NIHY with CSB. NIE of nifedipine (0.1-3 μg) was not influenced in IS or NIHY. Nitroglycerin (0.3-10 μg) had little effect on CF in these conditions. These results suggest that the decrease in P02alone does not affect CF of PM and that verapamil may exert more potent NIE in ischemic areas of the heart than in normal.

On the action of nifedipine under conditions of variable stimulation patterns and [Ca2+]0 in guinea-pig atrium

The effect of nifedipine is studied in perfused isolated guinea-pig left atrium at a temperature of 37 degrees C and a stimulation rate between 60 and 240/min. Rhythmical stimulation at 240/min was interrupted by interposed stimulus intervals (test intervals) in a range of 0.1-4 s. The pressure amplitudes developed during subsequent test contractions rise up to a maximum with increasing test intervals. This time course is described as restitution in contractile response. Results. Restitution is somewhat faster in earlier than in later stages of perfusion. A rise in [Ca2+]0 accelerates restitution without changing maximal pressure development. Lowering [Ca2+]0 depresses pressure amplitudes over the whole range of test intervals. Nifedipine slows down restitution but has (in concentrations up to 10(-6) M) no effect on the maximum achieved after long test intervals following stimulation at 240/min. The negative inotropic effect of nifedipine on rhythmical pressure development rises with increasing stimulation frequency in the range from 60-240/min. An elevation in [Ca2+]0 does not reestablish control pressure-frequency relationship. Conclusions. In respect to the restitution process and also to the pressure-frequency relationship the effect of nifedipine can neither be simulated by a reduction in [Ca2+]0 nor abolished by an elevation in [Ca2+]0.