948BOUCEK 17. Hey EN 1969 The relation between environmental temperature and oxygen consumption in the newborn baby. J Physiol (Lond) 200:589 18. Jammes Y. Auran Y. Gouvemet J, Delpierre S. Grimand C 1979 SummaryThe effects of postnatal development on the systolic and diastolic responses to pharmacologic blockade of the slow inward calcium current were investigated in blood-perfused hearts is* lated from immature (14-21-day-old) and adult rabbits. Isovolumic left ventricular develowd pressure. resting Dressure. and maximal rate of pressure dkve~d~ment ( + d~/ d t J s t cumulative doses of either verapamil, nifedipine, or diltiazem were determined by means of an intracavitary balloon. Myocardial contractile function in the immature heart was more sensitive to pharmacologic blockade of the slow inward calcium current than is the adult heart. Doses of verapamil, or nifedipine, that comparably reduced pretreatment developed pressure and +dP/dr were approximately 10-fold less in immature as compared to the adult heart. The dose of diltiazem which reduced developed pressure and dP/dt by 50% was 3-fold less in immature as compared to the adult heart. Verapamil and nifedipine decreased resting pressure in the adult but not in the immature heart. Conversely, diltiazem decreased resting pressure in the immature while not affecting resting pressure in adult hearts. Thus, postnatal cardiac development affects both the systolic and diastolic responses to Received January 6, 1983; accepted February 8, 1984. Vol. 18. No. 10, 1984 Printed in U.S.A.calcium channel blockade. In addition, diltiazem appears to be qualitatively and quantitatively different from verapamil and nifedipine with respect to the age-related cardiac effects of calcium channel blockade.Abbreviations SR, sarcoplasmic reticulum SL, sarcolemma LV, left ventricularIn the mammalian heart, excitation is coupled to contraction by intracellular Ca cycling mediated by the concerted functions of the SR and SL (5). Several lines of evidence suggest that the relationship b e t w e e i~~ and SR in the regulation -df excitationcontraction coupling may undergo significant changes during postnatal development.Ultrastructural evidence suggests significant postnatal development of the SR and T-tubule network (13,16,24,25). Ca-"triggered" Ca release by SR can first be demonstrated in skinned cardiac fibers 48 h after birth (3, 4). There is also suggestive evidence that the Ca reuptake properties of SR, important in relaxation, also develops after birth (16,22,28). Postnatal differences in cardiac contractile responses to extracellular Ca (23,26), lanthanum (7), manganese (7), and caffeine (1 3), tension development under voltage-clamp conditions (16) suggest that: (a) a new source of "activator" Ca develops with the postnatal development of the SR and T-tubule system; and (b) trans-sarcolem-