The action of amlodipine on voltage‐operated calcium channels in vascular smooth muscle

Abstract

1 Amlodipine, a dihydropyridine derivative largely ionized at physiological pH, inhibited calcium channel currents in single vascular smooth muscle cells isolated from rabbit ear artery in a concentration-dependent manner. 2 Amlodipine inhibited the current-voltage relationship for calcium channel currents across the range of test potentials used. However, the effect of amlodipine was more marked on more depolarized test potentials. Amlodipine also shifted the steady-state inactivation curve for calcium channel currents in a hyperpolarized direction. 3 The potency of amlodipine as determined from the steady-state inhibition of calcium channel current induced by the drug was dependent on the holding potential of the cells. Use of a more depolarized holding potential increased the potency of amlodipine. 4 Onset of amlodipine-induced inhibition was relatively rapid at both -60 mV and -40 mV holding potential. The use of a more depolarized holding potential increased the rate of association of amlodipine. No recovery from amlodipine-induced inhibition was seen over a 20 min period following washout of the drug. 5 In addition to voltage-dependence, the action of amlodipine showed use-dependence, in that the effect of amlodipine was more marked when calcium channel currents were evoked frequently. Increasing the frequency of activation of calcium channel currents did not alter the apparent onset rate of amlodipine-induced inhibition, but increased the degree of inhibition achieved by the drug. 6 The electrophysiological properties of amlodipine, particularly its voltage-dependence are probably important determinants of its action in vivo.

Binding constants determined from Ca2+ current responses to rapid applications and washouts of nifedipine in frog cardiac myocytes.

“…Whether this discrepancy arises from tissue differences or from an inhomogeneity in the functional state of the cells remains unclear. Additional information was provided by measuring the effects of two other DHP antagonists, (-)-202 791 ) and amlodipine (Hughes & Wijetunge, 1993), on Ica in single rabbit ear artery cells. In these experiments, depolarization of the membrane potential was accompanied by an increase in k, with little effect on k1c Hughes & Wijetunge, 1993).…”

“…Additional information was provided by measuring the effects of two other DHP antagonists, (-)-202 791 ) and amlodipine (Hughes & Wijetunge, 1993), on Ica in single rabbit ear artery cells. In these experiments, depolarization of the membrane potential was accompanied by an increase in k, with little effect on k1c Hughes & Wijetunge, 1993). Our analysis using a two-state model provides two sets of values for k1 and k1c depending on whether the Ca2P channel is in its resting or inactivated state.…”

“…-80 mV to values ranging from -40 to -20 mV, was observed in cardiac (Sanguinetti & Kass, 1984;Bean, 1984;Gurney et al 1985; Yatani et al 1986;Kokubun et al 1986;Hamilton et al 1987;Wei et al 1989;Herzig et al 1992), skeletal (Cognard et al 1986), and smooth muscle cells (Hughes & Wijetunge, 1993), as well as in non-muscle cells (Yaney et al 1991;Liu et al 1994), for a number of DHP antagonists. These include nitrendipine (Bean, 1984), isradipine (Cognard et al 1986;Kokubun et al 1986;Hamilton et al 1987;Wei et al 1989;Yaney et al 1991;Liu et al 1994; but see Lee et al 1987), nifedipine (Sanguinetti & Kass, 1984;Gurney et al 1985;Herzig et al 1992), nicardipine (Sanguinetti & Kass, 1984), nisoldipine (Sanguinetti & Kass, 1984), amlodipine (Hughes & Wijetunge, 1993), nimodipine ), (+)-Bay K 8644 , and (-)-202-791 . Using the analysis of Bean (1984), the apparent affinity Kapp obeys the relationship:…”

Effect of hypercholesterolaemia on voltage-operated calcium channel currents in rabbit arterial smooth muscle cells

“…16 Arteries isolated from WHHL showed no visible evidence of atheromatous changes. Vessels from both strains were cut into short strips (2-3 mm) and incubated in a lowcalcium physiological salt solution containing (mM): NaCl 130, KCl 6, CaCl 2 0.01, MgCl 2 1.2, glucose 14 and HEPES 10.7 buffered to pH 7.4 with NaOH and 2 mg/ml bovine serum albumin, 1 mg/ml collagenase, 0.5 mg/ml papain and 5 mM dithiothreitol for 50-60 mins at 37°C.…”

“…The experiments were carried out in 'high-barium solution' containing (mM): BaCl 2 110 and HEPES 10 buffered to pH 7.2 with TEA-OH to increase the size of the inward current elicited by depolarization and to minimise calcium-dependent inactivation of currents. 16 Voltage-clamp command pulses were provided by a PC via a Labmaster A/D interface board using commercially available software (PClamp 5.5, Axon Instruments, CA, USA). Data were recorded on-line after analogue-to-digital conversion at between 2.5-10 kHz depending on the voltage protocol used.…”