1. The subcellular distribution and maturation of Ruthenium Red-insensitive Ca2+ transport activity were determined in livers of rats ranging in age from 3 days pre-term to 10 weeks of adult life and compared with those of glucose 6-phosphatase, 5′-nucleotidase and Ruthenium Red-sensitive Ca2+ transport. Initial rates of Ruthenium Red-insensitive Ca2+ transport were highest in those fractions enriched in glucose 6-phosphatase, i.e. the microsomal fraction; this fraction was devoid of Ruthenium Red-sensitive Ca2+ transport activity. Although the heaviest fraction (nuclear) contained significant amounts of 5′-nucleotidase activity it was devoid of Ruthenium Red-insensitive Ca2+ transport activity. 2. Foetal rat liver contain minimal amounts of Ruthenium Red-insensitive Ca2+ transport activity, glucose 6-phosphatase and 5′-nucleotidase activities. These begin to be expressed concomitantly soon after birth; Ruthenium Red-insensitive Ca2+ transport is maximal by 3 to 4 days and remains so for up to at least 10 weeks of adult life. Glucose 6-phosphatase also reaches a peak at 3–4 days, but then rapidly decreases to approach adult values. Maximal activity of 5′-nucleotidase in the microsomal and nuclear fractions is seen about 4–6 days after birth; this enzyme activity remains increased for up to about 10 days and then falls, but not as rapidly as glucose 6-phosphatase. It is tentatively suggested that the bulk of the Ruthenium Red-insensitive Ca2+ transport is attributable to the system derived from the endoplasmic reticulum. 3. Administration of glucagon to adult rats enhances by 2–3-fold the initial rate of Ruthenium Red-insensitive Ca2+ transport in the intermediate but not the microsomal fraction. The hormone-induced effect is fully suppressed by co-administration of puromycin, is dose-dependent with half-maximal response at approx. 1μg of glucagon/100g body wt. and time-dependent exhibiting a half-maximal response about 1h after administration of the hormone. 4. Ruthenium Red-insensitive Ca2+ transport in the post-mitochondrial fraction of foetal liver also responds to the administration in situ of glucagon. The response, which also is prevented by co-administration of puromycin, is maximal in those foetuses nearing term. The suggestion is made that these effects of the hormone on Ruthenium Red-insensitive Ca2+ transport are an integral part of the physiological network in the liver cell.

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