Indications for vascular alpha- and beta-2 adrenoceptors in synapses of the muscarinic pathway in the pithed normotensive rat

In the pithed normotensive rat the adrenoceptors involved in the hypertensive and tachycardic effects of the indirectly acting sympathomimetic agent tyramine and of electrical stimulation of the spinal cord (TH5-L4 or C7-Th1) were analyzed. The tools used for the identification of the adrenoceptors were the selective alpha-1 adrenoceptor blocking drug prazosin, the selective alpha-2 adrenoceptor antagonist rauwolscine, the beta-1 blocker atenolol and the selective beta-2 adrenoceptor blocking agent ICI 118,551. The participation of vascular alpha-2 adrenoceptors in the pressor response of tyramine was shown. The increase in blood pressure induced by electrical stimulation of the spinal cord (TH5-L4) was used to demonstrate that alpha-2 adrenoceptors were activated via ganglionic muscarinic receptors. The tachycardia evoked by electrical stimulation of the spinal cord (C7-Th1) was not influenced by beta-2 adrenoceptor blockade. It was enhanced, however, by the alpha-2 adrenoceptor antagonist rauwolscine. It is hypothesized that activation of ganglionic nicotinic receptors leads to stimulation of the nearest varicosities interfering with alpha-1 adrenoceptors. However, activation of ganglionic muscarinic receptors may lead to an additional release of neurotransmitter in the more distant varicosities endowed with alpha-2 or beta-2 adrenoceptors.