Ionic basis for electrical properties of tonic fibres in rat extraocular muscles

1. The ionic conductances underlying some of the electrophysiological properties of multiply innervated or tonic fibres of rat extraocular muscles were examined in vitro with double-barrelled micro-electrodes.2. Exposure of the muscle to a Cl-free saline did not change the effective resistance (R(eff)) of tonic fibres which was 5.14 +/- 0.45 MOmega (n = 7) in control saline and 4.78 +/- 0.45 MOmega (n = 12) in Cl-free saline (P > 0.1). In contrast, in singly innervated or twitch fibres Cl removal increased R(eff) from 1.77 +/- 0.21 MOmega (n = 19) to 2.69 +/- 0.12 MOmega (n = 22) (P < 0.001).3. Tonic fibres with membrane potentials restored to - 80 mV by injecting current responded to intracellular depolarizing pulses with a brief, slow response (slow peak potential) which added to the rising phase of the electrotonic potential. The slow peak potential began at a membrane potential of - 40 to - 35 mV and was graded. Increasing depolarizations evoked faster and larger responses which did not over-shoot the zero level of membrane potential.4. The slow peak potential was not blocked by 10 muM-D-600 hydrochloride but was markedly reduced by the absence of Na and by 10 muM-tetrodotoxin. The response was broadened about five times by 25 mM-tetraethylammonium.5. Raising bath temperature from 21-25 degrees C to 37 degrees C reversibly depressed and shortened the slow peak potential but did not transform it into an action potential.6. It is concluded that the characteristic high R(eff) of tonic fibres results from a lack of a membrane conductance to Cl and that the slow peak potential involves the transient activation of Na and K channels which are pharmacologically similar to the respective channels of twitch fibres.