Enhanced testosterone secretion in adult rams after establishment of a high-frequency, low-amplitude pattern of LH pulses in the nonbreeding season occurs without changes in the number or binding affinity of testicular LH receptors

Abstract

When the LH signal in the ram is changed from one of large and infrequent pulses to one of small and frequent pulses, the testes quickly become more responsive to LH and testosterone secretion is elevated, perhaps because the number and (or) binding affinity of testicular LH receptors have increased. An experiment was undertaken in the nonbreeding season (July) with 10 adult Dorset \m=x\Leicester \m=x\Suffolk rams that were about 3.5 years of age and 69 \ m=+-\2 kg in body weight. Rams were given injections into the jugular vein of either 5 \g=m\g NIH\x=req-\ LH-S24 (in 1 ml saline) or vehicle every 80 min for 6 days. LH treament produced a series of LH pulses that occurred three times more frequently and were 70% less in amplitude than pulses in the control rams, without causing mean LH concentration to increase. Endogenously produced LH pulses were not evident in the treated rams after LH injection began. The modified LH-pulse pattern elevated mean testosterone concentration by 150% (assessed on days 2 and 5), and caused the cumulative testosterone response to LH pulses, estimated by multiplying testosterone-pulse amplitude by frequency per 6 h, to increase progressively by 180% (days \m=-\2 through 5). Enhanced testicular steroidogenic activity, presumably due to greater enzymatic activity and cholesterol availability within Leydig cells, was not associated with increases in either the concentration or affinity of LH\x=req-\ binding sites in the testis (assessed on days 3 and 6).The testes of adult rams secrete four to five times more testosterone in the breeding season than in the nonbreeding season (1, 2). Increased testos¬ terone secretion in the breeding season is the result of high-frequency, low-amplitude LH releases stimulating highly responsive testes (3). Increases in the number of Leydig cells (4), and (or) in the number of LH receptors per Leydig cell (5) could explain why the small LH pulses of the breeding season are able to trigger very large elevations in testosterone secretion. The LH-pulse pattern itself seems to be involved in maintaining testicular re¬ sponsiveness in that the elevation in testosterone secretion in response to an LH release is propor¬ tionately greater when LH releases are small and frequent (6).We hypothesized that establishing a high-fre¬ quency, low-amplitude pattern of LH pulses in rams would enhance the responsiveness of the testes to LH by increasing the number and (or) binding affinity of LH receptors. In a short-term study in the nonbreeding season, adult rams were given a series of LH injections to produce an LHpulse pattern resembling that of the breeding season. Changes in the pattern of testosterone se¬ cretion were assessed in relation to possible changes in gonadotropin receptors within the testis, as well as in FSH and PRL secretion which in rodent spe¬ cies also affect testosterone secretion (7,8).

Prolactin regulation of testosterone secretion and testes growth in DLS rams at the onset of seasonal testicular recrudescence

“…In the third collection (day 5), 5 mg of NIH-oLH-S24 in 1.0 ml of saline were injected into the jugular vein immediately after the sixth, seventh, and eighth blood samples were taken. The intent was to produce a prolonged rise in the blood LH with an amplitude similar to that of the episodic LH releases that occur during testicular recrudescence (Sanford & Baker 1990, Sanford et al 1993. The LH challenge probably caused maximal or near maximal stimulation of the testes.…”

“…Unoccupied LH and FSH receptors were measured by specific binding of 125 I-labeled ovine LH and FSH to plasma-membrane fractions prepared from testicular tissue (Yarney et al 1988, Sanford & Baker 1990. The specific activities of the labeled hormone preparations were 50 mCi/mg for 125 I-oLH and 75 mCi/mg for 125 I-oFSH.…”