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by the response to depolarizing current pulses and by constant-current I--V plots in solutions in which all Ca ions had been replaced by Co. 5.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.
