The effects of tetracaine on the electrical and mechanical activity of the guinea-pig ureter smooth muscle.

The existence of two populations of potential-operated Ca 2+ channels in the ureter smooth muscle have been suggested: (1) inactivating Ca 2+ channels which are responsible for the generation of action potentials and phasic contractions, and (2) non-inactivating Ca 2+ channels opened during sustained depolarization which are responsible for the generation of tonic tension (Kochemasova and Shuba 1979; Aickin et al. 1984). Reuter and Seitz (1968) introduced the idea that Na-Ca countertransport might be responsible for Ca movement in cardiac muscle. Heart muscle is known to produce contractile responses to low-Na solutions, an effect thought to reflect the activity of Na-Ca exchange mechanism (Chapman 1974). Aickin et al. (1984) observed in Na-loaded ureter smooth muscle a low-Na contracture mediated by the Na-Ca exchange. Unlike potential-operated Ca 2+ channels, the Na-Ca exchange proved to be completely insensitive to organic Ca 2+ antagonists (Aickin et al. 1984). Our study was aimed at characterizing the effects of tetracaine on the action potentials and phasic contractions as well as contractures of the ureter muscle stimulated by high-K and Na-free solutions. Simultaneous electrical and mechanical records were obtained from isolated pieces of whole guinea-pig ureter using the double sucrose-gap method (Biilbring and Tomita 1969) and tension alone was recorded with the continuous superfusion technique (Brading and Sneddon 1980). Fig. 1 shows the inhibitory effects of 0.5 mmol/1 tetracaine on the evoked action potentials and phasic contractions of the ureter muscle. Dose-response curve of the tetracaine-induced inhibition of phasic contractions mediated by the action potentials is illustrated in Fig. 2 (curve d). IC 5 o for phasic contractions was 0.2 mmol/1. The experiments with high-K contractures showed that tetracaine had a stronger inhibitory effect on the depolarization-induced tonic tension (Fig. 2,