The purpose of this study was to investigate the mechanisms responsible for
ischemia-induced changes in spontaneous electrical activity. An ischemic-like Tyrode
solution (pH 6.6) reversibly depolarized the maximum diastolic potential (MDP) and reduced the action potential (AP) overshoot (OS). We used
SNARF-1, which is an
indicator of intracellular pH (pH(i)), and perforated-patch techniques to test the hypothesis that
acidosis caused these effects. Acidic but otherwise normal Tyrode
solution (pH 6.8) produced similar effects. Basic Tyrode
solution (pH 8.5) hyperpolarized the MDP, shortened the AP, and slowed the firing rate. In the presence of "ischemic" Tyrode
solution, hyperpolarizing current restored the MDP and OS to control values.
HOE-642, an inhibitor of Na/H exchange, did not alter pH(i) or electrical activity and did not prevent the effects of ischemic Tyrode
solution or recovery after washout. Time-independent net inward current but not hyperpolarization-activated inward current was enhanced by ischemic Tyrode
solution or by 30 microM BaCl(2), a selective blocker of inward-rectifying K currents at this concentration. The results suggest that 1)
acidosis was responsible for the
ischemia-induced effects but Na/H exchange was not involved, 2) the OS was reduced because of depolarization-induced inactivation of inward currents that generate the AP upstroke, and 3) reduction of an inward-rectifying outward K current contributed to the depolarization.