Some properties of Ca(2+)-induced Ca2+ release mechanism in single visceral smooth muscle cell of the guinea-pig

Abstract

1. Late transient outward Ca(2+)-dependent K+ current (ILTO) correlated with Ca(2+)-induced Ca2+ release mechanism was studied in relation to the calcium inward current (ICa) in single isolated smooth muscle cells of the guinea-pig ileum using the whole-cell patch-clamp technique. 2. The voltage dependencies of peak ICa and ILTO were both bell shaped. However, the I-V curve of the outward current was shifted toward more positive potentials by about 60 mV in comparison to that for ICa. 3. Reduction in the external Ca2+ concentration resulted in a decrease of peak amplitude of both ICa and ILTO. However, caffeine-induced outward current was also decreased abruptly suggesting a rapid loss of stored Ca2+ upon lowering the external Ca2+ concentration. 4. Investigation of the relation of ILTO to partially inactivated ICa showed that inactivation of ICa by approximately 65, 80 or 84% of control (produced by prepulse to -20 mV for 2 s, shifting the holding potential to -20 mV for 30 s or by the ramp voltage command from -50 to +10 mV, respectively) was without detectable effect on the ILTO generation. 5. Bath application of the Ca2+ antagonist nifedipine (300 nM) inhibited ICa by 81% without affecting ILTO peak amplitude (92.0 +/- 5.6% of control in six cells). The mean concentration-response curve for ICa inhibition was sigmoidal with the apparent dissociation constant of 86.9 nM, whereas that for the ILTO had a characteristic sharp transition indicating a definite threshold of Ca2+ influx for ILTO generation. 6. Application of Ca(2+)-free external solution during 500 ms of the time when ICa peaked inhibited the current by about 76% whereas the ILTO during such an intervention remained virtually unchanged. 7. In double-pulse experiments, with conditioning and test pulses to +10 mV from -50 mV and an interpulse interval of 600 ms, most of the cells (about 80%) showed larger outward current at the test pulse suggesting continued Ca2+ release triggered by Ca2+ influx during a short (50-200 ms) depolarizing prepulse. The outward current could also be evoked at large positive potentials (presumably near the calcium equilibrium potential) where it did not occur normally by a prepulse to +10 mV for 50 ms. The charge transferred by Ca2+ current necessary to activate Ca2+ release in most of the cells was estimated to be from 6 to 20 pC. 8. The data are interpreted to suggest that the Ca(2+)-induced Ca2+ release mechanism operates in single ileal cells in a regenerative manner.(ABSTRACT TRUNCATED AT 400 WORDS)