BORON, WALTER F. Intracellular pH transients in giant barnacle muscle fibers. Am. J. Physiol. 233 (3): C61-C73, 1977 or Am. J. Physiol.: Cell Physiol. 2 (2): C61-C73, 1977.-The intracellular pH (pHi) of giant barnacle muscle fibers was measured using pH microelectrodes. The mean, initial pHi of 43 fibers incubated in barnacle seawater (BSW) at pH 7.5 and 22 C was 7.31 t 0.01 (SE). During prolonged (> 30 min) exposure of a fiber to NH&l-BSW, pHi rapidly rose and then slowly fell (plateau-phase acidification), while returning the fiber to NH&l-free BSW caused pHi to undershoot its initial value. Both the plateau-phase acidification and the undershoot were reversed when the electrochemical gradient for NH,+ was inverted by depolarizing the fiber with high-K+ BSW. During prolonged exposure of a fiber to CO,-BSW, pHi rapidly fell and then slowly rose (plateau-phase alkalinization), while returning the fiber to COz-free BSW caused pHi to overshoot its initial value. Both the plateau-phase alkalinization and the overshoot were irreversibly blocked by 0.5 mM 4-acetamido-4’-isothiocyanostilbene-2, 2’-disulfonic acid (SITS). After a fiber was exposed to 50 mM NH&l-BSW(pH 7.70) for 30 min and then returned to NH&l-free BSW, pHi fell to about 6.7. After this acidification, pHi slowly rose when the fiber was in BSW buffered with HEPES, but recovered much more rapidly when the BSW was buffered with CO,/HCO,-. The recovery was inhibited irreversibly by SITS and by 1 mMp-isothiocyanatobenzenesulfonate and reversibly by 10 mM pyridoxal phosphate. Data from pHi changes produced by exposing fibers to NH&l and CO, were used to calculate intracellular buffering power over a range of values of pHi. ammonium chloride; carbon dioxide; 4-acetamido-4’-isothiocyanostilbene-2, 2’-disulfonic acid; intracellular buffering power; active transport of acid

IT HAS BEEN KNOWN for years that exposing cells to NH&l causes intracellular alkalinization (19)) whereas exposure to CO, produces intracellular acidification (18). Until the introduction of pH-sensitive microelectrodes, however, it was difficult to examine the transient changes of intracellular pH (pHi) which occur during such experiments. Recent microelectrode studies have demonstrated that squid axons exposed to NH&l undergo a rapid intracellular alkalinization which is followed (during a period termed the “plateau phase”) by a slower acidification (2). Removing the external NH&l causes pHi to return to a value less than the initial one (an “undershoot”). Other microelectrode experiments (2, 11, 26, 27) have shown that cells exposed to CO, undergo a rapid intracellular acidification which