Changes in CO2 or in pH modify airway smooth muscle contractility. To investigate the mechanisms involved, we compared K(+)-induced contractions in porcine bronchial rings exposed to different CO2 concentrations and directly measured the effects of changes in intracellular (pHi) or extracellular pH (pHo) on Ca2+ currents (ICa) through voltage-dependent Ca2+ channels (VDC) in porcine tracheal smooth muscle cells. Hypocapnia and hypercapnia caused leftward and rightward shifts, respectively, in the dose-response to K+ (P < 0.05) but did not change the maximum force obtained. Peak ICa (10 mM external Ca2+) elicited by depolarizing pulses from -80 mV was maximal [-265 +/- 12 pA (mean +/- SE), n = 19] at +10 mV. Intracellular acidification decreased the peak ICa at +10 mV from -261 +/- 20 pA to -177 +/- 12 pA (P < 0.05, n = 4), while intracellular alkalinization increased the peak ICa at +10 mV from -302 +/- 27 pA to -368 +/- 26 pA (P < 0.05, n = 4). Changes in pHo had little effect on ICa. There was no shift in the voltage-dependence of induced ICa with any change. We conclude that pHi, but not pHo, directly modulates the entry of Ca2+ into airway smooth muscle cells through VDC. This mechanism may contribute to regulation of airway tone by CO2.