Calcium and potassium systems of a giant barnacle muscle fibre under membrane potential control

Abstract

1. Single barnacle muscle fibres from Megabalanus psittacus (Darwin) were internally perfused and the effects of various internal and external solutions on voltage clamp currents were examined.

2. The usual internal solution was 180 mM-K⁺ aspartate (osmotic pressure adjusted to 1000 m-osmole by adding sucrose). Fibres perfused with this solution gave an average resting potential of -55 ± 5 mV (all potentials are referred to the external solutions as ground). Further increase in internal K concentration depolarized the fibres.

3. With membrane current control the total capacitance, referred to apparent membrane surface area, was 21·2 ± 2·4 μF/cm².

4. Under normal conditions, with demonstrably good longitudinal space clamp control, voltage clamp currents associated with certain depolarizing pulses showed oscillations. These oscillations were reduced in frequency and magnitude by lowering the temperature from 20 to 10° C, by eliminating the inward currents with external Ca-free saline or by reducing the outward currents with internal tetraethylammonium (TEA) or replacement of internal K by Cs.

5. With a Na- and Ca-free, 60 mM-MgCl₂ solution outside depolarizing voltage clamp pulses produced only outward currents. On repolarization the current tail reversed direction at about -70 mV for pulses of less than 50 msec duration. For longer pulses this reversal potential was less negative, suggesting an accumulation of external, or depletion of internal K.

6. Both the size of the outward currents and the rate at which they reached their maximum value increased with temperature. The activation energy for the rate constant was about 63 kJ/mole.

7. Fibres bathed in Na- and Mg-free, 60 mM-CaCl₂ saline were excitable. After replacement of the internal K⁺ with Cs⁺ or adding 60 mM-TEA to the internal solution only sustained inward currents were recorded with depolarization.

8. Sustained inward currents could be reduced by external application of 5 mM-LaCl₃. Tetrodotoxin was not effective even at a concentration of 1000 nM.

9. The rate at which these inward currents reached a maximum value increased with increase in temperature of the bathing solution with an activation energy of the order of 42 kJ/mole.

10. The reversal potential of the inward currents changed with the level of internal Ca ions. For a fibre perfused without ethyleneglycol-bis (β-aminoethyl ether) N,N′-tetraacetic acid (EGTA) this reversal potential was 175 mV (internal free Ca 5 × 10^-7 M), and was 196 mV for a fibre perfused with 20 mM-Tris EGTA (internal free Ca 0·26 × 10^-8 M).

11. We propose an electrical equivalent circuit to account for most of the observed electrical properties of barnacle muscle fibres. In this model the Ca and the K system are located at different anatomical places and they interact through a series resistance.