Comparison between K+-induced and LHRH-induced gonadotropin secretion in vitro

The effect of elevated external [K+] compared to luteinizing hormone-releasing hormone (LHRH) on gonadotropin secretion was studied in vitro in superfused pituitary fragments from cyclic female rats. Challenging proestrous but not estrous pituitaries with a 2-h pulse of 0.8 nM LHRH produces two distinct peaks in the curve for the first derivative of gonadotropin secretion versus time; the second peak exceeds the initial one and represents expression of LHRH self-priming. Challenging proestrous pituitaries with a 2-h pulse of raised [K+] results in an initial increase in secretion followed by a positive slope of the secretion rate curve for LH but a constant rate for follicle-stimulating hormone (FSH); the positive slope is blocked by cycloheximide and not found with estrous pituitaries pulsed with raised [K+]. Proestrous pituitaries exhibit a graded response to 10-min pulses of raised external [K+] from 10-91 mM for LH and 10-47 mM for FSH secretion. LHRH followed by elevated [K+] primed the LH and FSH response to the secretagogue, but the reverse sequence did not. The results show 1) a concentration dependence of elevated [K+]-induced LH and FSH secretion, 2) a LHRH specific initiation of priming that does not require the immediate presence of LHRH for its subsequent expression, and 3) a gonadotropin secretory response to elevated [K+] that varies during the estrous cycle and can be modified by protein synthesis inhibition or progesterone administration.