Effect of immobilisation on the catecholamine- and histamine-induced responses of blood pressure and pulse rate in rats adapted to swimming and in controls.

Blood pressure and pulse rate responses to noradrenaline, isoproterenol and histamine were studied in restrained albino rats adapted to swimming and in controls. In the early phase of immobilization blood pressure remained stable but fell then abruptly to a lower level. The pulse rate showed a time-related exponential decline which was of lesser degree in the animals adapted to swimming. The amplitude of the response to noradrenaline increased during the four hours of immobilisation, in opposition to the histamine and isoproterenol-induced responses which showed no time-related changes. No differences between the trained animals and the controls were demonstrable for noradrenaline and histamine, while the isoproterenol-induced responses differed also during immobilisation. The comparatively slow decline of pulse rate in the trained animals is primarily attributed to the increased resistance of the trained heart, and the difference in the isoproterenol-induced responses is connected with an enhanced responsiveness of the beta-adrenergic receptors of the trained organism.