The physiological role of posterior pituitary alpha-melanocyte stimulating hormone in ultraviolet radiation-induced skin pigmentation.

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Using ultraviolet (UV) -sensitive male hairless mouse ; (HR-1xHR/De) F1, this study was conducted to investigate the physiological role of posterior pituitary alpha-melanocyte stimulating hormone (alphaMSH) in UV radiation (UVR) -induced skin pigmentation.The results of in vivo and in vitro experiments are as follows : (1) either posterior pituitary lobectomy (LOBEX) or hypophysectomy reduced plasma alphaMSH levels to approximately 20% of control (SHAM ; sham operation) values and completely abolished UV-induced skin pigmentation, (2) administration of alphaMSH to these groups restored skin response to UVRs with normalized plasma alphaMSH levels, (3) epidermal tyrosinase activity (TA ; a rate limiting enzyme for melanin synthesis) showed a positive-correlation to plasma alphaMSH levels, (4) TA in SHAM,LOBEX + alphaMSH and LOBEX groups increased remarkably following UVRs, although TA in LOBEX mice was only 20% of SHAM group, (5) plasma alphaMSH levels did not alter following a single or … More multiple UVR, (6) treatment of bromocriptine (BROM ; a dopamine agonist), similarly to LOBEX,decreased plasma alphaMSH levels but increased basal TA significantly compared to that in SHAM group, (7) BROM-treated mice also showed skin pigmentation by UVRs with similarly increased post-UVRs TA to SHAM, (8) dopamine inhibited alphaMSH secretion in a dose-dependent manner from the cultured posterior pituitary cells, and (9) alphaMSH,BROM and UVR increased TA in cultured skin tissue, however co-incubation with 10^<-7> M of neither alphaMSH nor BROM potentiated stimulatory effects of UVR on TA.These data suggest that circulating alphaMSH is derived mainly from the posterior pituitary and participates in UVRs-induced skin pigmentation through epidermal basal TA.However, it is likely that unidentified factors and / or mechanisms may induce skin pigmentation following UVR rather than mediation by circulating alphaMSH,since UVR did not alter alphaMSH secretion from the posterior pituitary and had stimulatory effects on TA in the cultured skin tissues under alphaMSH free condition. Less