The effect of vasopressin on extracellular cation concentrations and muscle resting potentials in the rat.

1. The Na+ and K+ concentrations in plasma and cerebrospinal (c.s.f.), resting potentials in skeletal muscle fibres, cardiac beat to beat intervals and 90% repolarization times were measured in Long Evans rats (parent strain controls) and in Brattleboro rats with hypothalamic diabetes insipidus (DI). 2. Cation concentration measurements confirmed previous observations that Brattleboro DI rats are mildly hypokalaemic compared with rats of the parent Long Evans strain, and indicated that the c.s.f. [Na+] is significantly raised in the former group of animals while the [K+] in the c.s.f. samples is similar in the two groups. 3. The mean resting potential of deep skeletal muscle fibres in Brattleboro DI rats was significantly more negative than the corresponding value in the Long Evans rats, and this finding was in close agreement with the difference observed for the calculated K+ equilibrium potentials in the two groups of animals. 4. The beat to beat intervals and the 90% repolarization times of cardiac action potentials were also determined in Brattleboro DI and Long Evans rats, and the mean values for both variables were significantly shorter in the former group of animals. 5. The administration of Pitressin by subcutaneous injection (500-100 mu./24 hr) to Brattleboro rats abolished the hypokalaemia and the hyperpolarization of skeletal muscle fibre membranes but had no significant effect on c.s.f. cation concentrations. 6. The present findings suggest that the absence of vasopressin in the Brattleboro DI rats results in a hyperpolarization of muscle cell membranes, and in changes in the cardiac action potential. These effects may be partly related to the mild hypokalaemia present in these animals.