Perinatal growth of the rabbit cardiac cell: Possible implications for the mechanism of relaxation

Abstract

Stereological analyses were performed on the right papillary muscle of fetal, newborn and adult rabbit heart and on the ventricular trabeculum of frog heart. Cell surface to volume ratio ( S V ) was high in the fetus (1.03 μm −1) comparable to that observed in the frog heart (1.32 μm −1) and remained stable in the newborn up to 8 days post-partum. At these ages there were no T-tubules. From 8 days post-partum onwards, cell diameter increased and S V ratio decreased progressively towards the adult value (0.30 μm −1 and 0.47 μm −1 including T-tubular area). A possible implication of the marked decrease of S V on cardiac mechanical activity was investigated. Unlike the relaxation of adult mammalian heart, the relaxation of the fetal heart was found to be sensitive to a reduction of NaCa exchange by low-Na solution as already shown in frog heart relaxation. During growth, the involvement of NaCa exchange in the relaxation of perinatal heart decreased and was progressively masked by a faster Ca-sequestration by the sarcoplasmic reticulum and/or other systems.