INTRACELLULAR H+ AND CA2+ MODULATION OF TRYPSIN-MODIFIED ATP-SENSITIVE K+ CHANNELS IN RABBIT VENTRICULAR MYOCYTES

ATP-sensitive K+ current (I(K.ATP)) channels are thought to play a role in the K+ efflux observed in cardiac ischemia. Intracellular acidosis is a prominent early effect in ischemia; therefore, the effects of acidosis on I(K.ATP) may have certain pathophysiological implications. Increased intracellular proton concentration (pH(i)) is known to regulate I(K.ATP) in frog skeletal muscle by increasing open probability. The pH(i) effect on I(K.ATP) is not clearly understood in heart because, unlike frog skeletal muscle, low pH(i) causes I(K.ATP) run-down in inside-out patches. This would tend to mask any opening effect of low pH(i) if it exists. Trypsin modification of I(K.ATP) has recently been shown to prevent run-down in inside-out patches. We used single channel recordings in inside-out patches to study I(K.ATP) after exposure to trypsin. After trypsin treatment, the open probability of I(K.ATP) was not sensitive to pH(i) in the absence of ATP. In the presence of ATP, however, a decrease in pH(i) consistently increased the open probability of trypsin-modified I(K.ATP) by reducing ATP inhibition. In the absence of ATP the mean open probability was 0.43+/-0.07 at pH(i) 7.4, and 0.5 mM ATP decreased the mean open probability to 0.03+/-0.04 at pH(i) 7.4, but mean open probability was significantly increased to 0.20+/-0.07 at pH(i) 6.3 (n=7, p<0.01). Ca2+ did not affect the activity of trypsin-modified I(K.ATP) in either the absence or presence of ATP at pH(i) 7.4. However, Ca2+ was able to antagonize the low pH(i) effect. A similar concentration of Mg2+ did not influence the low pH(i) effect, suggesting that the pH(i) effect does not depend on a particular proton-ATP complex. These results suggest that an intrinsic proton binding site regulates ATP sensitivity in cardiac I(K.ATP) channels. This may be a mechanism for K+ efflux via I(K.ATP) in early ischemia before ATP is severely depleted.