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 x Leicester x Suffolk rams that were about 3.5 years of age and 69 +/- 2 kg in body weight. Rams were given injections into the jugular vein of either 5 micrograms NIH-LH-S24 (in 1 ml saline) or vehicle every 80 min for 6 days. LH treatment 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 -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-binding sites in the testis (assessed on days 3 and 6).