Intracellular acidosis differentially regulates KV channels in coronary and pulmonary vascular muscle

Decreases in intracellular pH (pH(i)) potently dilate coronary resistance arteries but constrict small pulmonary arteries. To define the ionic mechanisms of these responses, this study investigated whether acute decreases in pH(i) differentially regulate K+ currents in single vascular smooth muscle (VSM) cells isolated from rat coronary and pulmonary resistance arteries. In patch-clamp studies, whole cell K+ currents were elicited by 10-mV depolarizing steps between -60 and 0 mV in VSM cells obtained from 50- to 150-mu m-OD arterial branches, and pH(i) was lowered by altering the NH4Cl gradient across the cell membrane. Progressively lowering pH(i) from calculated values of 7.0 to 6.7 and 6.4 increased the peak amplitude of K+ current in coronary VSM, cells by 15 +/- 5 and 23 +/- 3% but reduced K+ current in pulmonary VSM cells by 18 +/- 3 and 21 +/- 3%, respectively. These changes were reversed by returning cells to the control pH(i) of 7.0 and were eliminated by dialyzing cells with pipette solution containing 50 mmol/l HEPES to buffer NH4Cl-induced changes in pH(i). Pharmacological block of ATP-sensitive K+ channels and Ca2+-activated K+ channels by 1 mu mol/l glibenclamide and 100 nmol/l iberiotoxin, respectively, did not prevent changes in KC current levels induced by acidotic pH(i). However, block of voltage-gated K+ channels by 3 mmol/l 4-aminopyridine abolished acidosis-induced changes in K+ current amplitudes in both VSM cell types. Interestingly, alpha-dendrotoxin (100 nmol/l), which blocks only select subtypes of voltage-gated K+ channels, abolished the acidosis-induced decrease in K+ current in pulmonary VSM cells but did not affect the acidosis-induced increase in K+ current observed in coronary VSM cells. These findings suggest that opposing, tissue-specific effects of pH(i) on distinct subtypes of voltage-gated K+ channels in coronary and pulmonary VSM membranes may differentially regulate vascular reactivity in these two circulations under conditions of acidotic stress.