The effects of moderate, chronic (5 days) potassium depletion on cardiac function were assessed in 14 normokalemic and 13 hypokalemic open chest, anesthetized dogs. Cardiac responses to intravenous bolus injection of 2.5 micrograms/kg body weight epinephrine (10 normokalemic and 11 hypokalemic dogs) and to rapidly increased preload (8 dogs in each group) were evaluated. Hypokalemic dogs received a low potassium diet plus chlorthalidone. Plasma potassium levels were lower (p less than 0.001) in the hypokalemic dogs (3.2 +/- 0.1 mEq/liter [mean +/- SEM]) than in the normokalemic dogs (4.1 +/- 0.1). The inotropic response to epinephrine was lower in hypokalemic than in normokalemic dogs, the response of the maximal rate of rise of left ventricular pressure was 20% greater (p less than 0.03) and the response of the peak rate of change of ejection power was 60% greater in the normokalemic dogs. The relaxation response to epinephrine (the maximal rate of fall of left ventricular pressure) was 33% lower (p less than 0.02) in hypokalemic dogs. Responses to rapid volume expansion were impaired by hypokalemia; maximal stroke volume index was 31% lower (p less than 0.01), maximal cardiac index was 26% lower (p less than 0.01) and the peak response to the maximal rate of filling was 51% lower (p less than 0.01). There were no differences in basal cardiac function. Therefore, modest potassium depletion within the clinical range impaired the contractile and relaxation responses to epinephrine and preload and impaired rapid filling.

To test the hypothesis that the anestrous increase in estradiol negative feedback prevents estrous cycles by suppressing hypothalamic gonadotropin-releasing hormone (GnRH) pulse frequency, a variety of regimens of increasing GnRH pulse frequency were administered to anestrous ewes for 3 days. A luteinizing hormone (LH) surge was induced in 45 of 46 ewes regardless of amplitude or frequency of GnRH pulses, but only 19 had luteal phases. Estradiol administration induced LH surges in 6 of 6 ewes, only 3 having luteal phases. Anestrous luteal phase progesterone profiles were similar in incidence, time course, and amplitude to those of the first luteal phases of the breeding season, which in turn had lower progesterone maxima than late breeding season luteal phases. In the remaining ewes, progesterone increased briefly or not at all, the increases being similar to the transient rises in progesterone occurring in most ewes at the onset of the breeding season. These results demonstrate that increasing GnRH pulse frequency induces LH surges in anestrus and that the subsequent events are similar to those at the beginning of the breeding season. Finally, they support the hypothesis that the negative feedback action of estradiol prevents cycles in anestrus by suppressing the frequency of the hypothalamic pulse generator.

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