Gradation of Ca(2+)-induced Ca2+ release by voltage-clamp pulse duration in potentiated guinea-pig ventricular myocytes.

1. This study tests the hypothesis that whole-cell cardiac SR Ca2+ release is graded by recruitment of independent 'release units'. Structurally, an individual release unit may comprise ca four sarcolemmal L-type Ca2+ channels, adjacent ryanodine-sensitive sarcoplasmic reticulum (SR) Ca2+ release channels and the junctional gap between them. After depolarization, the first opening of a single L-type Ca2+ channel of the unit provides sufficient Ca2+ influx to increase local [Ca2+] beyond the threshold activating Ca(2+)-induced Ca2+ release (CICR), which amplifies local [Ca2+] until all release channels of the unit are active. This all-or-none activation does not spread to other release units. Gradation of whole-cell Ca2+ release is predicted to correlate with the cumulative probability density distribution of first latency of L-type Ca2+ channels or the activation time course of the calcium current, ICa. 2. Guinea-pig ventricular myocytes were potentiated by paired voltage-clamp pulses (1 Hz, 2 mM [Ca2+]o, 40 microM K5-indo-1, 36 degrees C). When the cellular Ca2+ load was at a steady high level, cytosolic calcium concentration ([Ca2+]c) transients were measured in response to test pulses of varied pulse duration (PD, 1-180 ms) and amplitude (-20, 0, 20 and 50 mV). The maximal rate of rise (RRmax) of the [Ca2+]c transient was used as an indicator for SR Ca2+ release. 3. Fast [Ca2+]c transients due to 4 ms pulses to 0 or 50 mV were blocked by 1 mM cadmium suggesting that these Ca2+ release signals are triggered by Ca2+ influx through L-type Ca2+ channels and not by Ca2+ influx through Na(+)-Ca2+ exchange. 4. RRmax increased with longer PD along the sigmoidal curve [1-exp(-PD/tau)]kappa(exponent k: 2 < k < 3). The time constant, tau, resembled the activation time constant of whole-cell ICa (Cs(+)-dialysed cells). A PD longer than a limiting duration did not modify RRmax. That is, inactivation of ICa was not reflected in the duration dependence. 5. Single L-type Ca2+ channels (cell-attached patches, 36 degrees C, -20 mV, 3.6 mM CaCl2 and 1 microM Bay K 8644 in patch pipette) opened with a waiting time the cumulative probability distribution of which resembled the duration dependence of RRmax, suggesting that the first opening of L-type Ca2+ channels determines whether the corresponding release unit contributes to the [Ca2+]c transient activated during a short voltage-clamp pulse. 6. The time constant, tau, of the duration dependence was shorter at positive than at negative potentials.(ABSTRACT TRUNCATED AT 400 WORDS)