Local suppressive effect of clonidine on penile erection in the dog

Dogs, 8.5 to 10 kg. in weight, were anesthetized with sodium pentobarbital (35 mg./kg.), intraperitoneally. Penile erection as indicated by an increase in the intracorporal pressure (ICP-increase) was produced by electrical stimulation of the right cavernous nerves. Drugs were administered into the internal pudendal artery (IPA) and femoral vein. A low dose (0.2 to 0.4 microgram/kg.) of clonidine, an alpha 2 adrenoceptor agonist, which could not affect either ICP or systemic arterial pressure (SAP) through an intravenous route, did suppress the ICP-increase markedly via direct injection into the IPA which supplies the penile blood flow. By intra-IPA injection, yohimbine (2.5 micrograms/kg.), an alpha 2 adrenoceptor antagonist, remarkably restored the ICP to the erection state. By intravenous injection, clonidine at a dose of 1.6 to 3.2 micrograms/kg. also profoundly reduced the ICP-increase, but only negligibly lowered the SAP. The IPA blood flow (IPAF) decreased coincidentally when the ICP-increase was effectively reduced by either intravenous or intra-IPA injection of clonidine. These findings suggest clonidine could act locally in the penile structure to suppress penile erection, possibly resulting from a penile vasoconstriction involving alpha 2 adrenoceptor. Whether this vasoconstriction is caused by a direct alpha 2 stimulating effect on the vascular smooth muscle or by an alpha 2 presynaptic inhibition of the vasodilator nerve (cavernous nerve) endings has been discussed.