INTRACELLULAR PH MODULATES THE AVAILABILITY OF VASCULAR L-TYPE CA2+ CHANNELS

L-type Ca2+ channel currents were recorded from myocytes isolated from bovine pial and porcine coronary arteries to study the influence of changes in intracellular pH (pH(i)). Whole cell I-Ca fell when pH(i) was made more acidic by substituting HEPES/NaOH with CO2/bicarbonate buffer (pH(o) 7.4, 36 degrees C), and increased when pH(i) was made more alkaline by addition of 20 mM NH4Cl. Peak I-Ca was less pH(i) sensitive than late I-Ca (170 ms after depolarization to 0 mV). pH(i) effects on single Ca2+ channel currents were studied with 110 mM BaCl2 as the charge carrier (22 degrees C, pH(o) 7.4). In cell-attached patches pH(i) was changed by extracellular NH4Cl or through the opened cell. In inside-out patches pH(i) was controlled through the bath. Independent of the method used the following results were obtained: (a) Single channel conductance (24 pS) and life time of the open state were not influenced by pH(i) (between pH(i) 6 and 8.4). (b) Alkaline pH(i) increased and acidic pH(i) reduced the channel availability (frequency of nonblank sweeps). (c) Alkaline pHi increased and acidic pHi reduced the frequency of late channel re-openings. The effects are discussed in terms of a deprotonation (protonation) of cytosolic binding sites that favor (prevent) the shift of the channels from a sleepy to an available state. Changes of bath pH(o), mimicked the pHi effects within 20 s, suggesting that protons can rapidly permeate through the surface membrane of vascular smooth muscle cells. The role of pH(i) in Ca2+ homeostases and vasotonus is discussed.