STIMULATION-INDUCED POTENTIATION OF T-TYPE CA2+ CHANNEL CURRENTS IN MYOCYTES FROM GUINEA-PIG CORONARY-ARTERY

1. Whole-cell Ca2+ channel currents were studied in myocytes isolated from guinea-pig circumflex coronary artery at 36-degrees-C and with 10 mM-Ba2+ (or Ca2+) as charge carrier. With 180 ms clamp steps from the holding potential of -100 mV, currents at -30 mV were carried mostly through the T-type calcium channels while at positive potentials currents were mostly of the L-type. 2. The increase in frequency of pulsing from 0.1 to 2.5 Hz resulted in a reduction of peak inward current ('negative staircase') with the 180 ms pulses to + 10 mV, but in a 2-fold potentiation ('positive staircase') with pulses to -30 mV. T-type currents and their frequency-mediated potentiation did not change significantly when Ba2+ was substituted by Ca2+ or Sr2+. 3. Potentiation of T-type currents was further analysed with a paired-pulse protocol: at a basal frequency of 0.1 Hz, a pre-pulse (inducing current I1) was followed by a 200 ms repolarization to -100 mV and a test pulse (inducing current I2). The potentiation could only be recorded using test pulses depolarizing the membrane to potentials between -40 and -10 mV; at more positive test potentials it was masked by the depressant effect of pre-pulses on the L-type current. 4. Potentiation of I2 by 200 ms pre-pulses started at pre-pulse potentials more positive than -60 mV and saturated at -20 mV (I2 potentiated by a factor 2.4). Between -20 and + 130 mV the potentiation was not dependent on the pre-pulse potential suggesting that the influx of Ba2+ or Ca2+ is not required for this effect. Potentiation of I2 by a 10 s pre-pulse followed the voltage dependence of the steady-state inactivation curve of the T-type Ca2+ channel; potentiation became visible at potentials more positive than -80 mV and saturated at about -50 mV. 5. When changing the interval between two identical 200 ms pulses, the T-type current was found to recover completely from inactivation within 40 ms at -100 mV; at intervals of 160-320 ms maximal potentiation of I2 occurred. 6. With pre-pulses shorter than 200 ms, potentiation became attenuated when inactivation became less complete. When the potential during the interval between the pulses was -80 instead of -100 mV, maximal potentiation was reduced (I2 potentiated by a factor of 1.3 instead of 2.2) and occurred later (1.28 s). 7. Potentiated T-type currents inactivated faster. In the double-pulse experiments when peak ratio (I2/I1) was 1.98 +/- 0.17, the current during the second pulse showed a single exponential time course of inactivation with a time constant of 5.6 +/- 0.7 ms, while the current during the first pulse had a time constant of inactivation of 14.5 +/- 1.5 ms. Due to faster inactivation the second current (I2) transported nearly the same charge compared to first current (I1) (charge ratio 0.94 +/- 0.16). Unlike the current amplitude, the transported charge did not exhibit a transient potentiation when the pulse interval was increased. 8. At -100 mV, T-type Ca2+ channels recovered faster from steady-state inactivation than L-type Ca2+ channels. 9. It is concluded from the present experiments that stimulation-mediated potentiation of T-type calcium channels is a potential-dependent process. Potentiation of T-type Ca2+ channels is discussed in the context of single-channel data from the literature. It is suggested that the channel passes transiently during recovery from inactivation through a closed state from which it can be open with a first latency shorter than the first latency in the steady state. Potentiation of peak current can be attributed to a larger number of channel openings within a shorter period of time.