Effects of short photoperiod on ATPase activities in the testis of the immature Siberian hamster

The effects of changes in photoperiod length upon body weight; spleen, thymus, and testis weights; testis protein content; testis cation pump enzyme activities; and plasma testosterone were studied in the developing Siberian hamster, Phodopus sungorus. Male hamsters were exposed to a cycle of 16L:8D (long-day), until Day 18 when half were switched to a 10L:14D (short-day) cycle, until killed 0, 2, 4, 7, 10, 12, or 15 days later. Body weight and relative testis weight (expressed as percentage of body weight) increased steadily during the first week of exposure. After 10 days, the long-day hamsters consistently weighed more (p < 0.05). Relative testis weights in the short-day group began to decrease (p < 0.005) within 10 days and continued to decline. Testis homogenate K⁺-pNPPase- (as a measure of Na⁺,K⁺-ATPase) and Mg²⁺-pNPPase-specific activities closely paralleled testis weight, with the short-day animals (p < 0.05) differing after 10 days. Plasma testosterone levels remained below adult levels through exposure Day 15, but were relatively lower (p < 0.05) in the short-day group after 10 days. Spleen weights were similar for the long- and short-day groups. The short-day group had larger thymus weights after 12 days (p < 0.05), but thymus enzyme activities did not differ between the two groups. We conclude that cation pump activities in the Siberian hamster testis are significantly affected by changes in photoperiod length.