Response of the perifused anterior pituitaries of rats to synthetic gonadotrophin releasing hormone: A comparison with hypothalamic extract and demonstration of a role for potassium in the release of luteinizing hormone and follicle-stimulating hormone

Anterior pituitaries (AP) of adult male rats were perifused in vitro with medium 199 and exposed to hypothalamic extract (HE) or to synthetic gonadotrophin releasing hormone (SGnRH). These stimuli were infused either as 10 min pulses at hourly intervals or continuously over a 5-h period. The levels of LH and FSH in 10 min collections of effluent were measured by radioimmunoassay. Except for a somewhat slower return to baseline with SGnRH, the AP’s pattern of response to pulsatile stimulation by the two agents was similar. The releasing potency of 10 ng of SGnRH was comparable to that of one hypothalamic equivalent (1 HEq). The response to pulses of varying concentrations of HE was linear over a range of 0.06 HEq to 8 HEq/ml. The linear range for SGnRH was from 0.6 to 20 ng/ml. The amounts of LH and FSH released by pulses of 20, 40 and 80 ng/ml SGnRH were comparable, indicating a plateauing of the response. When 2, 4, or 8 HEq/ml were added to the 80 ng/ml pulses of SGnRH, additional increments in the LH and FSH release occured in a dose-related fashion and with a slope similar to that obtained with HE alone from the same pituitary. Addition of KC1 to 80 ng/ml pulses of SGnRH, in amounts corresponding to those present in 2, 4, or 8 HEq/ml also caused further increases in LH and FSH release above the maximum that could be achieved by 20-80 ng/ml pulses of SGnRH. Addition of the amount of K⁺ present in 2HEq to 1 ml of medium 199 raised the concentration of K⁺ in the medium by about 27%. These amounts of KC1 alone did not cause any release of LH or FSH. Equimolar amounts of NaCl added to pulses of SGnRH also increased LH release but not in a dose-related fashion and they suppressed FSH release. The release of LH and FSH during continuous infusion of either HE or SGnRH was not constant. However, it was not possible to establish the extent to which the deviation from constant release was due to increase in responsiveness of AP during the period of stimulation or to pulsatile release. When pituitaries were stimulated with pulses of SGnRH in high concentrations, there was an indication of increased responsiveness after the first 3–4 pulses.