TABLE II .

Effect of Muscle pH on Action Potential Parameters in Muscle Fibers from Adult Rats at 4 and 9 mM K⁺

[K+]o	pH	n	Rheobase	Rate of rise	Peak potential
			nA	V/s	mV
4	7.4	4/38	57 ± 2	142 ± 9	6 ± 2
4	6.8	4/36	42 ± 2b	178 ± 9b	14 ± 1b
9	7.4	4/22	–	39 ± 5	−12 ± 2
9	6.8	4/38	–	56 ± 5a	−5 ± 1b

To measure action potentials, the current and the voltage electrode were placed 0.3 mm apart in the same fiber, and by injecting depolarizing constant current pulses through the current electrode, action potentials were elicited and recorded by the voltage electrode. pH was lowered from 7.4 to 6.8 by increasing CO₂ from 5 to 24%. To determine the rheobase current in muscles at 4 mM K⁺, 25-ms pulses with increasing strength (steps of 5 nA starting at 10 nA) were injected until an action potential was elicited. In muscles at 9 mM K⁺, only a single depolarizing pulse of 100 nA was injected. In the excitable fibers (see text), the rate of rise of the action potential was calculated from the slope of the action potential upstroke between 10 and 90% of the action potential amplitude. n gives muscles/fibers used in each group. Values are means ± SEM.

^aP < 0.05 (significantly different from corresponding value at the same [K⁺]_o and normal pH).

^bP < 0.01 (significantly different from corresponding value at the same [K⁺]_o and normal pH).