抄録
Effects of external pH on the electrical and mechanical properties were investigated on the bullfrog atrium under voltage-clamped and unclamped conditions with double-gap method in order to elucidate the principal site of pH action. On rising the pH (5.3-8.3) a marked positive inotropic effect and an increase in rate of rise of action potential were produced with a shortening of the plateau duration. The potassium contracture was also augmented. Under voltage-clamped conditions, fast (If) and slow inward currents (Is) as well as delayed outward current were enhanced indicating a definite action of pH on the surface membrane. Is-dependent and independent contractile tensions in response to a small (70mV, 0.2sec) and large depolarizing pulses (200mV, 1.0 sec) were both strengthened.
Excess caffeine (20mM) Ringer solution augmented the effects of pH, while Ca-free La (1mM) Ringer solution abolished, suggesting a minor role of the sarcoplasmic reticulum in the pH effects. A contracture in Na-free high Ca (80mM) solution was enhanced by lowering external pH. This effect was eliminated after DNP (2mM) which affects the mitochondria.
These results indicate that the changes in external pH act mainly on the sarcolemma and a rise of pH produces the positive inotropic effects by enhancing the Ca-influx due to Is as well as Na-Ca exchange mechanism, although the mitochondria tended to show a minor counter action.
