title: Alpha-drenergic inhibition of calcium-dependent potentials in rat sympathetic neurones.
abstract: 1. Post-ganglionic neurones of the rat superior cervical ganglion were studied in vitro (21-26 degrees C) using single intracellular micro-electrode methods. 2. Three Ca2+-dependent potentials were studied: the shoulder on the normal action potential, the hyperpolarizing afterpotential (h.a.p.), and th Ca2+ spike. 3. Bath-applied noradrenaline reversibly inhibited these Ca2+-dependent potentials. The EC50 for inhibition of peak h.a.p. amplitude was about 1 microM. The order of catetholamine potency was: L-adrenaline > L-noradrenaline > D-noradrenaline congruent to dopamine > DL-isoprenaline. Phentolamine (10 microM), an alpha-blocker, but not MJ-1999 (10 microM), a beta-blocker, antagonized the action of noradrenaline. 4. Noradrenaline (10 microM) hyperpolarized most neurones (1-6 mV) studied, with no detectable change in resting membrane conductance. 5. Superfusion with low external Ca2+ and high Mg2+ mimicked the effect of noradrenaline. Either procedure alone antagonized the h.a.p. conductance increase but did not alter the h.a.p. reversal potential. However, in the presence of low Ca2+, high Mg2+, the remaining action potential and h.a.p. were not further reduced by noradrenaline. 6. The Ca2+-dependent shoulder of the action potential did not appear dependent upon GK. Noradrenaline and low Ca2+ antagonized the shoulder when enhanced by TEA+ or Ba2+. 7. Both the rate of rise and amplitude of the Ca2+ spike were antagonized by noradrenaline. 8. We propose that activation of an alpha-adrenoceptor inhibits a voltage-sensitive Ca2+ conductance (GCa(V)), thereby reducing the inward Ca2+ current which may generate the noraml action potential shoulder and the rising phase of the Ca2+ spike. Reduction of Ca2+ current would also reduce the Ca2+-dependent portion of outward K+ current underlying the h.a.p.
mesh_terms: D000818:Animals; D001338:Autonomic Fibers, Postganglionic / Q000502:physiology*; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D004298:Dopamine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D004837:Epinephrine / Q000494:pharmacology*; D005728:Ganglia, Sympathetic / Q000502:physiology*; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D010646:Phentolamine / Q000494:pharmacology; D051381:Rats; D011941:Receptors, Adrenergic / Q000502:physiology*; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D013757:Tetraethylammonium Compounds; D001464:Barium; D013779:Tetrodotoxin; D007545:Isoproterenol; D002118:Calcium; D004298:Dopamine; D004837:Epinephrine; D010646:Phentolamine