Abstract
1. In pithed cats, the spinal sympathetic outflow was stimulated preganglionically at segments C7 and T1 and heart rate responses and nictitating membrane tone were measured in parallel. 2. Clonidine and a related drug, BS 100-141 (N-amidino-2(2,6-dichlorophenyl)acetamide hydrochloride), caused a dose-dependent inhibition of the stimulation-induced tachycardia but did not inhibit responses of the nictitating membrane. The inhibition of heart rate was antagonized by the alpha-adrenoceptor blocking drug, phentolamine. 3. In isolated hearts of rabbits, noradrenaline release in response to adrenergic nerve stimulation was reduced by clonidine and BS 100-141 and the effect was antagonized by phentolamine. 4. The results support the view that presynaptic alpha-adrenoceptors are involved in the regulation of transmitter release from adrenergic nerves. Cardiac adrenergic nerves appear more sensitive to alpha-adrenoceptor-mediated inhibition of inpulse transmission than the sympathetic nerves to the nictitating membrane.
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