Tiapamil reduces the calcium inward current of isolated smooth muscle cells. Dependence on holding potential and pulse frequency.

The calcium currents (ICa) of isolated smooth muscle cells (urinary bladder of the guinea-pig) were analyzed at 35 degrees C and 3.6 mM [Ca]0. The whole cells were voltage-clamped with a single patch electrode which was filled with 150 mM CsCl in order to block potassium outward currents. Tiapamil reduced ICa at concentrations between 1 microM (threshold) and 0.5 mM (complete block). Administration of 10 microM tiapamil at rest reduced ICa by 10% ('initial block'). Repetitive depolarizations (140 ms long pulses to -5 mV, applied at 1 Hz) reduced ICa further in a beat-to-beat fashion. This 'conditioned block' developed with a faster time course and to a greater extent when the stimulation frequency was increased and when the holding potentials were set to more positive levels. Reduction of ICa by tiapamil was facilitated by more positive and attenuated by more negative holding potentials. The influence of holding potential and pulse frequency on the tiapamil effect is discussed in terms of the 'modulated receptor hypothesis'.