EFFECT OF TILTING ON THE PRESSOR RESPONSES TO McN‐A‐343, A MUSCARINIC SYMPATHETIC GANGLION STIMULANT

1 In anaesthetized cats and dogs treated with mecamylamine, the pressor response to McN‐A‐343 was increased when the animals were changed from a supine to a head‐up, tilted position. This potentiation was not seen in rats. 2 The potentiation of the McN‐A‐343 pressor response was not affected by propranolol, destruction of the brain, or removal of the intestines, spleen or adrenal glands. It was promptly abolished by applying pressure to the lower half of the tilted animal. No increase in the pressor response to McN‐A‐343 occurred when cats were tilted head down. The potentiated response in tilted cats was abolished by atropine. 3 The pressor effects of adrenaline, noradrenaline, tryamine and angiotensin in cats treated with mecamylamine were either reduced or unchanged when the animal was changed from the supine to the tilted position. In one cat not treated with mecamylamine in which orthostatic hypotension occurred, tilting potentiated the pressor responses to dimethyl phenylpiperazinium iodide. 4 In cats anaesthetized with chloralose, the reflex pressor response to bilateral carotid occlusion was reduced by tilting. After mecamylamine treatment the residual atropine‐sensitive response to carotid occlusion was potentiated when the animal was placed in the tilted position. 5 These results suggest that muscarinic stimulation of sympathetic ganglia by McN‐A‐343 raises blood pressure by predominantly reducing venous capacity, in contrast to noradrenaline and angiotension which increase blood pressure mainly by arterial vasoconstriction. 6 It is not clear whether this is a general property of sympathetic ganglionic stimulation or is restricted to stimulation of muscarinic sites.