Differences and similarities among circadian characteristics of plasma renin activity in healthy young women in Japan and the United States.

A circadian rhythm of plasma activity (PRA) was demonstrated for both Japanese and North American women, the latter mostly Caucasians of mixed ethnic origin. The results were based on blood samples withdrawn at 4-hour intervals during a 24-hour span (in March 1978) from 20 subjects from Fukuoka (average age 20.4 +/- 0.1 years) and 16 subjects from Minneapolis (average age 20.2 +/- 0.4 years). The rhythms in the two populations showed similarities in some characteristics and differences in others. The timing of high values, i.e., of acrophases, objectively assessed by curve-fitting (and of corresponding 95 per cent confidence limits) was at 07(36) (05(00), 10(16) and 06(32) (03(00), 10(00) for Japan and USA, respectively. As objective measures of the extent of predictable rhythmic change mean amplitudes, in nanograms per milliliter per hour (ng/ml/hour), were similar (0.31 and 0.32); a statistically significant difference (P less than 0.05) was found in mean amplitudes expressed as percentage of the rhythm-adjusted average. Mean rhythm-adjusted average values (mesors) were lower in women from Japan than in those from the United States: (1.64 +/- 0.14 and 2.39 +/- 0.23 ng/ml/hour, respectively; P less than 0.01). A statistically significant difference in dietary salt, indicated by differences between the Japanese and North American women in the urinary excretion of sodium and chloride (P less than 0.05), almost certainly contributed to these results.