GTP modulates run-up of whole-cell Ca2+ channel current in a Ca(2+)-dependent manner.

1. Whole-cell voltage-clamp recordings were obtained from CA1 neurons acutely dissociated from rat hippocampus to study the effects of guanosine 5'-triphosphate (GTP) on the gradual increase in Ca2+ channel current amplitude that takes place over several minutes after breaking in to whole-cell mode ("run-up"). 2. Including GTP (500 microM) in the patch pipette significantly prolonged the duration of run-up of peak Ca2+ channel current to its maximum value compared with controls without GTP when the recording solutions contained Ca2+. On the other hand, GTP significantly enhanced run-up when Mg2+ and Ba2+ were substituted for intracellular and extracellular Ca2+, respectively. 3. The enhancement of run-up of the current in the Mg/Ba condition appeared to be due both to an initial increase in current amplitude that was complete within 30 s after break in and to a more rapid initial rate of run-up when compared with the Ca2+ condition. GTP did not affect the absolute maximum amplitudes of the currents in either Ca2+ or Ba2+ conditions. 4. We conclude that an early GTP-dependent modulation of Ca2+ channel current is qualitatively altered, depending on whether Ca2+ or Ba2+ is used as the charge carrier. Evidence of this modulation is apparent within seconds after rupture of the membrane patch. Conceivably, influences occurring during the period of "equilibration" with electrode contents could alter subsequent regulatory steps.