Effects of blood pressure manipulations on shivering thermogenesis in the pigeon

The effects of blood pressure (BP) manipulations on shivering thermogenesis were studied in conscious pigeons. A rise in BP induced by noradrenaline (NA) or equipotent doses of angiotensin II (Ang II) effectively suppressed shivering at +12 degree C and partly abolished the cold-induced vasoconstriction in the feet. The inhibition commenced when the rise in BP reached +40 mmHg, and a fall in body temperature followed these responses. Comparison of the trajectories in the BP-shivering plane revealed that the inhibition of shivering by Ang II could be completely explained by changes in BP, whereas NA had also another, more prolonged inhibitory action independent of baroreceptor activity. A similar dose-dependence for effects on BP and shivering could be established with both drugs. An acute reduction of BP by sodium nitroprusside had also a very potent inhibitory effect on shivering. In hypotensive pigeons elevation of BP with NA initially enhanced shivering, but when BP was raised beyond normal levels shivering was again suppressed. We conclude that both a rise and a fall in BP can inhibit shivering in the pigeon, and that normal levels of BP facilitate shivering. NA inhibits shivering by more than one mechanism, but the initial effect is mediated through a baroreflexive action. The interactions of thermoregulatory and cardiovascular mechanisms suggest an integrated control of body temperature and circulation, which should be considered in experimental approaches to these homeostatic systems.