Lifelong variations in heart rates in SPF Sprague Dawley rats of both sexes. Statistical correlations with body weights.

In unanesthetized Sprague Dawleys, SPF, of both sexes, housed in DL12:12 (100lux), at a temperature of 18--23 degrees C and a hygrometry of 60--75%, the measurements of heart frequencies during most part of their life point out a continuous decrease with age and an always higher (40--20 c.min-1) heart rate in females than in males. Significant correlations between heart rate and body weight can account for these heart rates variations. Between the ages of 72 and 589 days, which corresponds to the growth period, a linear relationship was established between Log heart rate (y; c. min-1) and Log body weight (x; gram); for males: Log y = 0.122 Log X +Log 938 and for females: Log y = -0.166 Log X + Log 1217. After the age of 600 days, which corresponds to senescence, decreases in heart rate as well as in body weight were observed in both sexes.