Effects of postnatal maturation on postrest potentiation in isolated rabbit atria

Abstract

Ultrastructural changes in cardiac sarco­plasmic reticulum (SR) have been reported during postna­tal development of the mammalian heart, but the functional significance of these observations has not been well char­acterized. Calcium release from SR in intact myocardial preparations was determined by the contractile character­istics of postrest contractions. Isometric tension and the maximum rate of tension development of the first contrac­tion following intervals of electromechanical quiescence (rest) were related to steady-state tension and maximum rate of tension development during contraction at constant frequency (1.0 Hz) in isolated left atrial strips from new­born (1-7 days), immature (14-21 days), and adult (more than 6 months) rabbits. The first postrest contraction was increased as a function of the rest interval rate of tension development, defined as postrest potentiation, in all three age groups and reached a maximum value at rest intervals of more than 20 s. Tension developed by the first contrac­tion following a 60-s rest interval was potentiated less in newborn and immature atria than in adult atria at an extracellular calcium concentration ([Ca]e) of 2.5 mM, an age-related difference most marked in the immature. Ry­anodine (5.0 x 10-9 M), a putative blocker of calcium release from cardiac SR, abolished postrest potentiation providing evidence that calcium release from SR is the predominant determinant of the postrest contraction. Post-rest tension in atria from the immature rabbit heart was significantly increased both in absolute terms and relative to steady-state tension following stabilization under con­ditions which increase intracellular [Ca] ([Ca]i), i.e. increas­ing [Ca]e, increasing tonicity, or decreasing extracellular sodium concentration ([Na]e). In contrast, postrest tension in atria from the adult rabbit heart decreased relative to steady-state tension under conditions that increase [Ca]j. Rest potentiated contraction to a similar extent in both age groups (approximately 300% of steady-state) but this max­imum was obtained under markedly different ionic condi­tions. Maximum postrest potentiation occurred at 2.5 mM [Ca]e and 153.0 mM [Na]e in the adult atria whereas maximum potentiation occurred at 5.0 mM [Caje and 87.0 mM [Na]e in immature atria. Results from exper­iments in which [Ca]e was increased during a 60-s rest interval suggest that the predominant effects of [Ca]e on postrest potentiation are attributable to [Ca]e-dependent changes in [Ca]i as opposed to rest-dependent changes in transmembrane calcium conductance. These results lead us to conclude that in rabbit atria: 1) contractile calcium released from the SR is demonstrated and therefore may potentially participate in excitation-contraction coupling during postnatal development; 2) the contribution of cal­cium released from the SR to contractile calcium during steady-state contraction may be less during early cardiac development; 3) the age-related differences in postrest contraction can be abolished by manipulating ionic condi­tions to increase [Ca]i indicating that the immature SR requires higher [Ca]i for optimal calcium loading and/or "triggered" release of calcium than does SR in the adult. © 1987 International Pediatric Research Foundation, Inc.