Appearance of calcium action potentials in crayfish slow muscle fibres under conditions of low intracellular pH

1. The intracellular pH (pHi) of crayfish slow flexor muscle fibres was measured using recessed-tip pH micro-electrodes. To study the electrophysiological effects of intracellular acidification, pHi was lowered to values between the normal 7 . 2 and 6 . 3 by removal of external NH4Cl after a 20--30 min exposure. 2. During intracellular acidification, the fibres became capable of generating all-or-none Ca action potentials, rather than the normal small graded responses; no change in resting potential or input resistance accompanied this change in excitability. 3. All-or-none spikes appeared at pHi = 6 . 4--6 . 5. The spikes disappeared when pHi was increased again following the addition of 10 mM-bicarbonate to the external solution. CO2-saturated saline, which decreased pHi to 6 . 4, also caused the appearance of all-or-none action potentials. 4. The appearance of action potentials was correlated with a decrease in delayed rectification as pHi fell, as indicated by the response to depolarizing current pulses and by constant-current I--V plots in solutions in which all Ca ions had been replaced by Co. 5. External tetraethylammonium (TEA), at concentrations which reproduced the pHi = 6 . 4 effects on the I--V relation, also caused the appearance of all-or-none action potentials. 6. It is concluded that low pHi in these fibres partially blocks (or causes a depolarizing shift in the voltage dependence of) the voltage-sensitive outward K current which normally shunts the inward Ca current and prevents the generation of action potentials.