TEMPERATURE EFFECTS ON IN-VITRO SKIN DARKENING IN THE MOUNTAIN SPINY LIZARD, SCELOPORUS-JARROVII - A THERMOREGULATORY ADAPTATION

The in vitro sensitivity of integumental pigment cells (melanophores) to darkening hormones was studied in a winter-active montane lizard (S celoporus jarrovii) that rapidly changes color. The darkening response s, shown by dose-response curves, of skin samples to melanophore-stimu lating hormone (alpha-MSH) were slightly higher at 15 degrees than at 25 degrees C, but not significantly so. At 35 degrees C, as compared t o 25 degrees C, the darkening responses were dramatically and unexpect edly reduced, a sevenfold higher concentration of alpha-MSH being need ed to elicit equivalent responses. Darkening responses to isoprotereno l, a beta-adrenoceptor agonist, applied to skins in the presence of th e a-adrenoceptor blocker Dibenamine, were significantly diminished at both 15 degrees and 35 degrees C, as compared to 25 degrees C. The sim ilarity of the dose-response curves for alpha-MSH at 15 degrees and 25 degrees C (no reduction in sensitivity at the lower temperature) and the marked decrease of response to alpha-MSH and isoproterenol at 35 d egrees C are consistent with the thermoregulatory chromatic behavior o f the animal dark color at low temperatures in order to maximize heat gain, and light color once high, activity body temperature is achieved . These in vitro results suggest a direct, temperature-dependent, cell ular response of S. jarrovii chromatophores to hormones. The temperatu re pattern suggests that the cellular response may be adaptive. Parall el experiments performed with Anolis carolinensis, a species that disp lays physiological color changes for crypsis rather than thermoregulat ion, demonstrated that the sensitivities to both alpha-MSH and isoprot erenol were significantly lower at 15 degrees and 35 degrees C, compar ed to 25 degrees C. Our results suggest that reptile pigment cell sens itivity to hormones at varying temperatures may be regulated at the ce llular level.