Adenylyl Cyclase of Perifused Porcine Granulosa Cells Remains Responsive to Pulsatile, but not Continuous Stimulation with Follicle-Stimulating Hormone*

Abstract

Differentiation of granulosa cells and development of ovarian follicles requires FSH for several days. The purpose of the present studies was to determine how the capacity of the adenylyl cyclase of immature porcine granulosa cells to respond to FSH depends on the dose, duration, and frequency of exposure to FSH. Cells were stimulated with various regimens of FSH and forskolin in a dynamic flow perifusion system. cAMP production during 4 h of continuous (tonic) exposure to FSH was directly related to FSH concentration (5-500 ng/ml). FSH-stimulated cAMP production declined markedly after 4 h of tonic stimulation with FSH, regardless of the FSH concentration. Preliminary experiments using a pulse interval of 3 h (perifusion medium FSH concentration, 150 ng/ml) indicated that stimulation with 15-min pulses elicited a greater cumulative cAMP response than stimulation with either 5-, 30-, or 60-min pulses. Responsiveness to FSH depended critically on the duration and frequency of stimulation and the concentration of FSH. Short pulses were more effective than long pulses in both eliciting cAMP responses of most uniform amplitude and maintaining responsiveness to a final tonic FSH stimulus. The optimal pattern of stimulation consisted of a pulse duration of 15 min, with a pulse interval of 2-3 h. A peak chamber FSH concentration of 150 ng/ml yielded the greatest cumulative cAMP production, although cells that had been perifused with FSH-free medium had the greatest response to a final tonic FSH stimulus. The attenuation of responsiveness after continuous perifusion with FSH does not appear to be due to desensitization of the cyclase itself, since 1) cells perifused with FSH continuously for 20 h still responded to forskolin (100 microM), which activates cyclase independently of the FSH receptor; and 2) cells did not become refractory to forskolin for 14 h. The transient refractoriness to FSH appears to be due to a process that alters the interaction between the FSH receptor and the guanine nucleotide regulatory component of cyclase. This refractoriness can be reversed simply by removing FSH from the perifusion medium for a critical period of time, i.e. 2-3 h.