Age-related changes in myocardial relaxation and sarcoplasmic reticulum function.

Rapid regulation of relaxation is essential to allow the heart to alter stroke volume in response to stress. Inasmuch as Ca2+ transport by the sarcoplasmic reticulum (SR) is an important determinant of relaxation, the purpose of this study was to examine developmental differences in the ability of isoproterenol to alter relaxation time and to determine if these differences were associated with age-related changes in Ca2+ transport by the SR in isolated, perfused adult and newborn guinea pig hearts. Control values of the time constant of isovolumic relaxation (tau) were 37.8 +/- 5.9 ms in adult hearts (n = 8) and 31.6 +/- 5.3 ms in newborn hearts (n = 6). With maximum isoproterenol stimulation, the decrease in tau was significantly greater in adult (51.1 +/- 8.8%, mean +/- SD) compared with that in newborn (26.3 +/- 3.1%, P less than or equal to 0.0001) hearts. Ca2+ uptake, Ca2(+)-dependent adenosinetriphosphatase activity, and Ca2+ pump density were all significantly greater in SR vesicles isolated from adult hearts compared with values measured in SR vesicles from newborn hearts. We conclude that developmental differences in the capacity of the SR to sequester Ca2+ may contribute to age-related differences in the functional response of the heart to isoproterenol.