Fig. 1. Dose-responses of various melanocyte cultures to -MSH. The responses of NHM 753-c, 830-c, 849-b and 755-c, homozygous or compound heterozygous for MC1R variants, and NHM 747-c, 729-c and 751-b, homozygous for the consensus MC1R were compared. The response of NHM 765-c, heterozygous for Val92Met substitution, is also presented, since it is comparable to NHM 753-c and 830-c in the ratio of eumelanin to pheomelanin. The responses to -MSH were tested as described in the Materials and Methods. The effects of increasing doses of -MSH on cAMP formation, tyrosinase activity and cell proliferation are presented in A, B and C, respectively. Basal levels of cAMP (Pmole/10⁶ cells) in the cultures tested were as follows: 753-c=2.176±0.134; 830-c=1.451; 849-b=1.172±0.0219; 755-c=2.193±0.206; 765-c=1.675±0.137; 747-c=1.074±0.098; 729-b=2.007±0.128; 751-b=0.84±0.103. In A and B, each value represents the mean percent of control of six determinations±s.e. In C, each value is the mean percent of control of three determinations±s.e.