MODULATION OF ATP-SENSITIVE K-SECRETING CELLS( CHANNELS BY INTERNAL ACIDIFICATION IN INSULIN)

The effect of intracellular acidification (low pH(i)) on open probabil ity of the ATP-sensitive K+ (K-ATP) channel was examined in insulin-se cretion cells using an inside-out configuration of the patch-clamp tec hnique. In an insulin-secreting cell line beta-TC3, K-ATP single-chann el currents (I-KATP) were readily recorded in the absence of internal ATP. ATP (50 mu M and 0.5 mM) dramatically decreased the channel activ ity. A step decrease of intracellular pH (pH(i)) from 7.4 to 6.7 or 6. 3 in the presence of ATP gradually increased the channel activity. In addition, low pH(i) in the presence of ATP could partially restore cha nnel activity lost in a process called ''rundown.'' Kinetic analysis r evealed a change in channel gating at low pH(i) with ATP. The bursting durations of I-KATP at pH(i) 6.3 in the presence of ATP were signific antly longer than those at pH(i) 7.4 in the absence of ATP. These resu lts suggest that the increased channel activity at low pH(i) might hav e resulted from a mechanism involving an alteration of channel conform ation. We also observed an inhibitory effect of low pH(i) on channel a ctivity. However, the inhibitory effect was much more apparent at pH(i ) 5.7 and was only partially reversible. The activation effect of low pH(i) on I-KATP in the presence of ATP was also observed in acutely is olated rat islet cells and in another insulin-secretion cell line RINm 5F, although the effect was weaker and was variable among experiments. We conclude that, as in frog skeletal muscle and cardiac muscle, an i ncrease in channel activity at low pH(i) is one of the mechanisms unde rlying proton modulation of I-KATP in insulin-secreting cells.