The influence of pH on the effects of organic anions in frog skeletal muscle
The depressant effect of acidity on twitches and K contractures in frog skeletal muscle was greatly accentuated in the presence of organic anions, particularly anions such as butyrate, which also reduced these responses at neutral pH. Conversely, alkaline pH antagonized the depression of contractile responses by butyrate. Most of the effects of acid pH were rapid in onset and were accomplished without any change in membrane resting or action potentials, although depolarization developed in the presence of carboxylate anions when pH was reduced below 6.0. Simultaneous variation in pH and butyrate concentration showed that the undissociated acid exerted a prominent depressant effect only when its concentration reached 1–10 mM, and that the marked depressant action of butyrate at neutral pH was produced primarily by the dissociated anion. Similar experiments showed that the dissociated anion also was largely responsible for the enhanced depolarizing effect of acidity in media containing carboxylates. Acid-induced depolarization was not facilitated in media containing methane sulfonate, but in spite of its low pKa, this anion also increased the sensitivity of contractile responses to the depressant effects of acidity. Hence, the accentuation of the effects of organic anions by acid pH must be exerted on the sequence of membrane events which link excitation and contraction. The effect of acidity was greater when longer apolar hydrocarbon chains were attached to the anionic group for both the carboxylate and the sulfonate series of ions. These depressant effects may be produced by interference with the membrane-stabilizing actions of divalent cations, and may involve increased membrane fluidity.