Voltage‐dependent decrease in the availability of single calcium channels by nitrendipine in guinea‐pig ventricular cells.

Abstract

SUMMARY1. The mechanism of Ca2+ channel block by nitrendipine was studied by recording single-channel activity from cell-attached patches on guinea-pig ventricular cells using patch pipettes containing 50 mM-Ba2+. Test depolarization pulses to around 10 mV with a duration of 100 ms were applied repetitively at 2 Hz.2. The percentage of non-blank sweeps was maximal (about 40%) at a holding potential between -65 and -130 mV and decreased sigmoidally with its depolarization. Nitrendipine shifted the availability-voltage relationship in a hyperpolarizing direction.3. From the number of consecutive non-blank sweeps and that of blank sweeps, the duration of the available state and that of the unavailable state were estimated.4. The histogram of the duration of the available state showed a singleexponential distribution. Its mean duration was about 1-5 s and was shortened by nitrendipine. Correspondingly, the decay of the mean current during the depolarization step was accelerated by nitrendipine.5. In the presence of 100 nM-nitrendipine the histogram of the duration of the unavailable state at large negative holding potentials was simulated as the sum of two exponential components, one with a time constant similar to that in the control and the other with a time constant of 6-7 s.6. The histogram of the duration of the unavailable state at depolarized holding potentials was simulated by a double-exponential curve also in the control. The duration of the slow component was prolonged by nitrendipine.7. The prolongation of the unavailable states initiated by drug binding during depolarization steps and maintained during depolarized holding potentials is the mechanism of the blockade. The rate constants of the state transitions between an available state and two unavailable states were estimated.

Single-Channel Pharmacology of Mibefradil in Human Native T-Type and Recombinant Cav3.2 Calcium Channels

“…This concentration caused qualitatively similar but smaller effects (I peak , from Ϫ93 Ϯ 24 fA to Ϫ69 Ϯ 24 fA; NP o , from 11 Ϯ 3.2% to 7.0 Ϯ 2.9%, p Ͻ 0.05, n ϭ 6). Selectivity of the action of 10 M mibefradil on these channels was further examined by applying the same concentration of nitrendipine (10.3 Ϯ 0.2 M, n ϭ 6), a selective blocker of single L-type calcium channels (Kawashima and Ochi, 1988). As expected, Ca v 3.2 channels were not blocked significantly by this compound ( Fig.…”

Independent Modulation of L-type Ca2+ Channel in Guinea Pig Ventricular Cells by Nitrendipine and Isoproterenol.

“…NIT accelerated I CaL decay during depolarization, as would be expected for a drug binding to opened channels. 11) In contrast, both ISO and FSK prolonged I CaL decay. When NIT and ISO or FSK were applied together, the ISO-or FSKinduced prolongation of I Ba decay was not observed (Figures 3 and 4).…”

“…Cell isolation: The methods used have been described previously. 11,15) Briefly, guinea pigs (250-350 g) were anesthetized with pentobarbital sodium (50 mg/kg) and the ascending aorta was cannulated under artificial ventilation. The heart was then excised and sequentially perfused by the Langendorff method with normal Tyrode solution (5 min), Ca -free (Kraftbrühe, KB) solution (5 min).…”

“…6) However, nicardipine, another DHP Ca 2+ channel blocker, is useful during this surgery to control preoperative and intraoperative hypertension. 7) Patch clamp studies have established that blockade of I CaL by a DHP Ca 2+ channel blocker is characterized by a negative shift in the steady-state inactivation and acceleration of I CaL decay during depolarization of cardiac muscle [9][10][11] and VSM. 12,13) On the other hand, β-agonist-induced increases in I CaL are associated with small, negative shifts in the current-voltage (I-V) relationship 14) and the slowing of voltage-dependent inactivation during depolarization in cardiac muscle.…”

Low K+‐induced hyperpolarizations trigger transient depolarizations and action potentials in rabbit ventricular myocytes

“…As described previously (Kawashima & Ochi, 1988), ventricular myocytes were isolated from the hearts of Japanese rabbits (1·6 kg) by an enzymatic treatment during Langendorff perfusion at 37°C. In short, rabbits were anaesthetized by intravenous injection of urethane (1·25 g kg¢) via the auricular vein; the ascending aorta was cannulated under artificial respiration; the heart was then excised and perfused sequentially with normal Tyrode solution for 5 min, Ca¥-free Tyrode solution for 5 min, collagenase solution for 20 min and Kraft-Br uhe (KB) medium for 5 min.…”