Effects of low pH on the action potential and contraction of normal and hypoxic papillary muscles

The effects of low pH (6.5) on action potentials and contractions of rabbit papillary muscles were examined under aerobic and hypoxic conditions (pO2 = 65- 75 mmHg, 8.6-10.0 kPa). An acid medium reduced overshoot, Vmax of phase O and action potential duration, without changing the resting potential, under aerobic aerobic conditions. Maximum twitch tension was reduced and resting tension was increased by 40% and 6% respectively of control values. Upon returning to normal Tyrode, all parameters returned to control values. The effects of acidosis under hypoxic conditions were essentially the same as those described above; acidosis potentiated the effects of hypoxia. It is concluded that in the given experimental conditions, low pH potentiates the effects of hypoxia on electrical and contractile activity, although probably by different mechanisms. The effects of acid medium on the action potential could be well accounted for the a reduction in slow inward current and by an increase in outward background potassium current, probably in relation to an increase in internal free calcium concentration.