Agradecimentos: This work was supported by FAPESP (Proc. N. 95/ 0355-3) and PADCT (Proc. N. 620123/94.0). We are indebted to Dr Nivaldo Baccan (Institute of Chemistry/UNICAMP) for determination of Ca2+ concentrations by atomic absorption spectrometry. We are also grateful to Mr Gilson B. Maia Jr, Mr Luis Henrique G. Ribeiro, Ms Karen A. Silva, Ms Caroline S. Sukushima and Mr Mateus Vercesi for technical assistance Ver menos

Abstract: Ca2+uptake, transmembrane electrical potential (??m) and oxygen consumption were measured in isolated ventricular mitochondria of rats from 3 days to 5 months of age. Estimated values of ruthenium red-sensitive, succinate-supported maximal rate of Ca2+uptake (Vmax, expressed as nmol Ca2+/min/mg protein) were higher in neonates and gradually fell during postnatal development (from 435±24 at 3–6 days, to 156±10 in adults,P<0.001), whereas K0.5values (10µm) were not significantly affected by age. Under similar conditions, mitochondria from adults (5 months old) and neonates (4–6 days old) showed comparable state 4 (succinate anda-ketoglutarate as substrates) and state 3ADP(a-ketoglutarate-supported) respiration rates, as well as ??m values (-150 mV). Respiration-indpendent ??m and Ca2+uptake, supported by valinomycin-induced K+efflux were also investigated at these ages. A transient ??m (-30 mV) was evoked by valinomycin in both neonatal and adult mitochondria. Respiration-independent Ca2+uptake was also transient, but its initial rate was significantly higher in neonates than in adults (49.4±10.0v28.0±5.7 mmol Ca2+/min/mg protein,P<0.01). These results indicate that Ca2+uptake capacity of rat cardiac mitochondria is remarkably high just after birth and declines over the first weeks of postnatal life, without change in apparent affinity of the transporter. Increased mitochondrial Ca2+uptake rate in neonates appears to be related to the uniporter itself, rather than to modification of the driving force of the transport Ver menos