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Centre de Recherches de Biochimie Macromoléculaire, CNRS UPR 8402, INSERM U 249, Montpellier, France.
1. The establishment of the whole-cell patch-clamp recording configuration (WCR) revealed a type of inhibition to which L-type Ca2+ channels were subject in static rat ventricular myocytes before obtaining the WCR. 2. Immediately after membrane disruption (< 10 s), the Ca2+ current (ICa) was absent but gradually increased in amplitude to reach its final waveform (amplitude and kinetics) 2-3 min after the WCR was reached. 3. Three distinct phases (P) were identified. First, no inward but an outward current, blocked (1-2 min) by Cs+ dialysing from the patch pipette (P1), was recorded. Second, overlapping with (P1), ICa increased dramatically to reach a maximum peak amplitude within 2-3 min (P2). Concomitantly, its rate of decay, initially monoexponential and slow, became biexponential owing to the appearance of a fast component of inactivation (P3). Complete interconversion between slow and fast components sometimes occurred. 4. Changes in current waveform were not related to voltage loss or series resistance variation, and suppression of an outward current (P1) was unlikely to account for P2 and P3. 5. The run-up of ICa was independent of the nature of the permeating ions, the membrane holding potential, depolarization, rate of stimulation, the intracellular Ca2+, ATP, Mg2+, Cs+ and the pH of the pipette solution. Since large Ca2+ currents were recorded using the perforated patch technique, the run-up of ICa is not explained by the wash-out of an inhibitory endogenous macromolecule during cell-pipette exchanges. 6. Pharmacological manipulations, including the use of Ca(2+)-Ba(2+)-EGTA and exposure of the cells to isoprenaline and/or Bay K 8644 prior to recording, did not alter the mechanism primarily responsible for build-up. Unrepriming of channel activity was required before these modulations could be effective. 7. Currents could however be instantly augmented when cells were extracellularly superfused during the run-up step. The wash-out of an inhibitory agent originating in the cell itself (such as H+, NH4+ and lactate) and accumulating in the extracellular microenvironment of the cells seems unlikely. Rather, we suggest that pressure-induced mechanostimulation may be involved in the restoration of Ca2+ channel activity.
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