Evidence for verapamil-induced functional inhibition of noradrenergic neurotransmission in vivo.
Contractions of the cat nictitating membrane have been used to explore the effects of calcium channel blockers on neurotransmission in vivo, by comparing the effects of verapamil and nifedipine on contractions of nictitating membrane following either electrical stimulation of the superior cervical ganglion or intravenous injection of the alpha-adrenoceptor agonist phenylephrine. Verapamil (0.3, 0.6 and 1.2 mg/kg, iv) produced a dose related and reversible inhibition of stimulation induced contractions but did not affect phenylephrine responses of nictitating membrane. Intravenous nifedipine (10, 20 and 40 micrograms/kg) produced inconsistent effects on both stimulation- and phenylephrine-induced contractions of the nictitating membrane. Thus only verapamil appears to selectively affect noradrenergic neurotransmission in this model, possibly by altering the neurotransmitter release from the terminals innervating the nictitating membrane in the cat.
Department of Pharmacology and Therapeutics, King George's Medical College, Lucknow, India.
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Gurtu
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Seth
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Roychoudhary
D000320:Adrenergic Fibers / Q000187:drug effects* / Q000502:physiology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D002415:Cats; D004558:Electric Stimulation; D005260:Female; D005728:Ganglia, Sympathetic / Q000187:drug effects / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009541:Nictitating Membrane / Q000187:drug effects / Q000294:innervation; D009543:Nifedipine / Q000494:pharmacology; D010643:Phenoxybenzamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D009435:Synaptic Transmission / Q000187:drug effects*; D014700:Verapamil / Q000494:pharmacology*
D000316:Adrenergic alpha-Agonists; D010643:Phenoxybenzamine; D010656:Phenylephrine; D014700:Verapamil; D009543:Nifedipine