Potassium and Calcium Currents in Dissociated Muscle Fibres of the Mollusc Philine Aperta

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Abstract
Dissociated unstriated muscle fibres from the buccal mass retractor muscles of the mollusc Philine aperta were studied using a two-electrode voltage-clamp. The mean resting potential of the fibres was −76.3±0.44mV (N=30), and the membrane resistance was 42.2±3MΩ. The space constant of the fibres was 2.03+0.33mm (N=5). Three outward potassium currents were resolved in response to a depolarising step to zero from resting potential. (1) An early transient current, voltageactivated and blocked by 2 mmoll−1 4-aminopyridine (4-AP). This resembled the A-current described in molluscan neurones and some arthropod muscle fibres. (2) A calcium-dependent late transient current, with slower kinetics, which was suppressed by 50 mmoll−1 tetraethylammonium chloride (TEA-Cl), zero-calcium saline, 1 mmol−1 Cd2+ and 1 μmoll−1 verapamil. (3) A delayed voltage-activated current, blocked by 50 mmoll−1 TEA-Cl and with kinetics associated with the delayed rectifier current IK. An inwardly directed current, blocked by zero-calcium salines, Cd2+ and verapamil, was considered to be a calcium current whose activation closely matched that of the Ca2+-dependent potassium current. A blockade of either the A-current, or exposure to low-calcium artificial sea water, or a combination of both, promoted the development of oscillations and regenerative spikes in the muscle fibre following depolarization