Antagonistic effects of vasopressin and hypervolemia on osmotic reactivity of the thirst mechanism in dogs

The reactivity of the thirst mechanism to osmotic stimuli was examined in conscious dogs 1. under control conditions, 2. after raising the plasma vasopressin (PADH) level to about 30 muU/ml by intravenous infusion of the hormone, 3. after expansion of the blood volume by 15% by an intravenous infusion of dextran solution, and 4. after a simultaneous increase of PADH and blood-volume expansion. The osmotic thirst threshold was significantly lowered by the elevation of PADH and augmented by an expansion of blood volume, whereas no significant changes were observed when the increase in PADH and expansion of blood volume were applied simultaneously. The interactions between body-fluid osmolarity, blood volume, and vasopressin in regulation of water intake are discussed.

Plasma catecholamines during sustained isometric exercise

1. Plasma catecholamine, haemodynamic and metabolic responses to sustained isometric exercise were studied in eight healthy subjects, who maintained handgrip at the 30% level of maximal voluntary contraction (MVC) for as long as possible.

2. The sustained handgrip was accompanied by a significant increase in plasma noradrenaline (NA) and adrenaline (A) concentrations.

3. The increase in plasma NA during handgrip was greater than that associated with heavy dynamic work involving large muscle groups.

4. The results suggest that the known haemodynamic responses to static effort are related to a powerful activation of the adrenergic system, which may result from a reflex mechanism initiated in the exercising muscles.