1. A modification of a previously published method for the disruption of the T‐tubules of frog skeletal muscle is described. The modification permits the disruption of the T‐tubules without the decline in resting potentials which was reported previously. 2. The method for the disruption of the T‐tubules involves the washout of glycerol following loading in a 400 m M glycerol Ringer solution. The modification consists of elevating the concentration of divalent cations in the Ringer used for glycerol washout. 3. The optimum concentrations are 5 m M‐Ca2+ and 5 m M‐Mg2+ added as their chloride salts. Neither 10 m M‐Ca2+ nor 10 m M‐Mg2+ are as effective as the combination of each at 5 m M. Other concentrations gave less satisfactory results. 4. The use of the modified technique provides a preparation which maintains 85–90 mV resting potentials for up to 6 or 8 hr but which will not contract in response to membrane depolarization.

The capacitance of skeletal muscle fibers was measured by recording with one microelectrode the voltage produced by a rectangular pulse of current applied with another microelectrode. The ionic strength of the bathing solution was varied by isosmotic replacement of NaCl with sucrose, the [K] [C1] product being held constant. The capacitance decreased with decreasing ionic strength, reaching a value of some 2 F/cm 2 in solutions of 30 mM ionic strength, and not decreasing further in solutions of 15 mM ionic strength. The capacitance of glycerol-treated fibers did not change with ionic strength and was also some 2 uF/cm 2 . It seems likely that lowering the ionic strength reduces the capacitance of the tubular system (defined as the charge stored in the tubular system), and that the 2 ptF/cm 2 which is insensitive to ionic strength is associated with the surface membrane. The tubular system is open to the external solution in low ionic strength solutions since peroxidase is able to diffuse into the lumen of the tubules. Twitches and action potentials were also recorded from fibers in low ionic strength solutions, even though the capacitance of the tubular system was very small in these solutions. This finding can be explained if there is an action potential-like mechanism in the tubular membrane.

Chloride current--voltage relations in Xenopus laevis muscle membrane have been investigated in phosphate-buffered solution (pH 5.2--5.4) using a three-microelectrode voltage clamp. Resting chloride conductance in these conditions is about 10(-4) S/cm2, approximately 1/10th that at pH 8.8. When the membrane potential is stepped from the holding (resting) potential to a more negative voltage the current rises from the initial to the steady state. The instantaneous current--voltage relation is linear and the steady-state relation shows inward-going rectification. As hyperpolarization appears to "activate" the chloride conductance, the "availability" of chloride current has been measured at the beginning of a voltage step to a standard test potential following conditioning at a variety of potentials. The relationship between the test current and the conditioning voltage is sigmoid. The normalized sigmoid curve has the same slope (absolute value) but opposite sign to that obtained in the same experiment at pH 8.8. In mildly acidic solutions (pH 6.4) the current wave form is diphasic: current initially falls then rises to the steady state. This combination of transients militates against the idea that transients are due solely to accumulation--depletion effects in restricted spaces ("unstirred layers") and a hypothesis is qualitatively outlined in which pH-and voltage-dependent effects are ascribed to a single type of channel whose orientation in the membrane is unconstrained.

SUBMARY1. The patterns of innervation and electrical properties of muscle fibres in a skeletal muscle of the blue tongue lizard Tiliqua nigrolutea have been investigated.2. Gold impregnation of nerve terminals and staining of muscle fibre junctional areas for cholinesterase showed that there are two histological types of muscle fibre in scalenus muscles of the lizard: (a) those usually receiving single en plaque innervation, and (b) those that receive multiple en grappe terminations.3. In normal solution and in solutions to which small doses of curare were added, two types of subthreshold post-junctional response were recorded following nerve stimulation, (a) potentials with rapid rates of rise and a half-decay time of less than 10 msec and (b) responses with fast rise times and long half-decay times (50 msec or more).4. Fast time course subthreshold responses often gave rise to propagated action potentials. In curarized preparations ((+)-tubocurarine 04-1-0 ,ug/ml.) action potentials failed, giving way to junction potentials of decreasing amplitude, when stimulation was maintained at rates of 5/sec or more. The decay phases of fast time course potentials were closely approximated by error functions.5. Slow time course responses summated during repetitive stimulation, but action potentials were never produced. The decays of slow junction potentials were well fitted by exponentials. It is suggested that fibres in which they were recorded received distributed, en grappe innervation.6. Fibres in which fast time course junction potentials were recorded were excited by direct stimulation via an intracellular micro-electrode. They had apparent membrane resistance and capacity of about 4000 Q. cm2 and 7,F/cm2. 7. Fibres exhibiting slow junction potentials could not be excited 32 Phy. I99

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