title: Slowing effects of dopamine and calcium-channel blockers on frequency of sodium spikes in rat pars intermedia cells.
abstract: 1. Spontaneous discharge of action potentials (Na spikes) in cells isolated from rat pars intermedia was slowed or arrested by Co(2+), Ni(2+) or Mn(2+), which block voltage-dependent Ca channels in these cells. The amplitude of persisting spikes was undiminished. The effects resembled those of dopamine.2. Action potential frequency decreased when the Ca(2+) concentration was lowered to 0.1 mM and increased when the Ca(2+) concentration was raised from this level to 1 mM or 2 mM or when Ba(2+) (2 mM) was introduced. These effects, together with those of Co(2+), Ni(2+) and Mn(2+), are consistent with the possibility that Ca(2+) participates in the regulation of spike discharge.3. Verapamil, methoxyverapamil (D600), and nifedipine reduced the amplitude of the individual Na spikes in concentrations that had little effect on voltage-dependent Ca channels. Action potential frequency was comparatively little affected by these drugs.4. K(+) (15 mM) stimulated action potential frequency and this effect too was suppressed by dopamine or Co(2+).5. The effect which dopamine had of slowing spontaneous discharge, like the inhibitory effect on secretion, was blocked by metoclopramide. But otherwise the mechanism is unclear: dopamine blocked voltage-dependent Ca channels in some cells but not in most others.6. The effects of K(+) and Ba(2+) of eliciting spikes, the suppression of Na-spike discharge by Co(2+) and related Ca-channel blocking cations, and the unspecific effects of the organic ;Ca channel blockers', all have implications for the use of these substances as tools to analyse stimulus-secretion coupling.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D002413:Cations, Divalent; D004298:Dopamine / Q000494:pharmacology*; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D008297:Male; D008787:Metoclopramide / Q000494:pharmacology; D009543:Nifedipine / Q000494:pharmacology; D010902:Pituitary Gland / Q000166:cytology / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D014700:Verapamil / Q000494:pharmacology
chemical_list: D002413:Cations, Divalent; D007473:Ion Channels; D000001:Calcimycin; D014700:Verapamil; D009543:Nifedipine; D008787:Metoclopramide; D011188:Potassium; D002118:Calcium; D004298:Dopamine