Double-pulse calcium channel current facilitation in adult rat sympathetic neurones.
1. Double-pulse facilitation of Ca2+ channel currents in enzymatically dispersed adult rat superior cervical ganglion neurones was investigated using the whole-cell variant of the patch-clamp technique. Voltage-clamp recordings were performed at room temperature (21-24 degrees C) in solutions designed to isolate Ca2+ channel currents. 2. Ba2+ currents, elicited by a 0 mV test pulse, were increased in amplitude when preceded by a 40 ms pulse to voltages greater than 0 mV. The magnitude of facilitation was dependent on pre-pulse voltage and reached a maximum of 50% (i.e. 1.5 x the current amplitude elicited without a pre-pulse) at a pre-pulse voltage of +80 mV. Half-maximal facilitation occurred at about +25 mV. A small decrease (-6%) in test pulse amplitude was present at pre-pulse voltages between -40 and 0 mV. The magnitude of facilitation was also dependent on test pulse voltage. Facilitation was greatest between test pulse voltages of -10 and 0 mV. 3. Facilitation slowly decreased during prolonged (1 h) dialysis of the neurone even though the Ba2+ current amplitude was well maintained. 4. Increasing the pre-pulse duration over the range 0-120 ms produced an exponential increase in facilitation with a time constant of 17.3 ms. Conversely, lengthening the interpulse duration over the range 5-915 ms, while maintaining a constant pre-pulse amplitude and duration, resulted in an exponential decrease in facilitation with a time constant of 197 ms. 5. At a test potential of 0 mV, the decay of the facilitated Ba2+ current component was fitted to a double exponential function with time constants of about 25 and 150 ms. The time constants had little pre-pulse voltage dependence between +30 to +80 mV. 6. The initial rising phase of both the control and facilitated Ba2+ current were reasonably well described by a single exponential (tau rise) after a delay of 300 microseconds. The tau rise versus test pulse potential relationship was 'bell shaped' over the test pulse voltage of -20 to +30 mV reaching a maximum near -5 mV. tau rise was similar for control and facilitated currents except at potentials greater than +10 mV where the rise of the facilitated current was accelerated. 7. Control and facilitated activation curves, as derived from tail current amplitudes, were described by the sum of two Boltzmann functions. A facilitating pre-pulse produced an increase in the proportion of the current contributed by the component activated at more hyperpolarized test potentials.(ABSTRACT TRUNCATED AT 400 WORDS)
Ikeda
S R
SR
Department of Pharmacology and Toxicology, Medical College of Georgia, Augusta 30912-2300.
D000818:Animals; D001464:Barium / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology*; D019204:GTP-Binding Proteins / Q000494:pharmacology; D005728:Ganglia, Sympathetic / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials; D008978:Mollusk Venoms / Q000494:pharmacology; D009431:Neural Conduction; D009539:Nicotinic Acids / Q000494:pharmacology; D010069:Oxadiazoles; D051381:Rats; D011919:Rats, Inbred Strains; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D008978:Mollusk Venoms; D009539:Nicotinic Acids; D010069:Oxadiazoles; D010566:Virulence Factors, Bordetella; C049834:PN 202-791; D001464:Barium; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D019204:GTP-Binding Proteins