Plasma clearance of heterogeneous growth hormone components in the rat: effects of diabetes and starvation.

Two forms of immunoreactive rat GH (rGH), bit rGH (b.rGH) and little rGH (l.rGH) are predominant in both pituitary and plasma from control (C), fasted (F), food-restricted (FR), diabetic (D) and insulin-treated D (D +l) rats. The l.rGH appears to be the predominant form in plasma and pituitary in all five experimental groups. However, in each group, the relative proportion of b.rGH in plasma exceeds from that in pituitary. To determine whether these differences are due to a delayed metabolic clearance of the big component, the MCRs and distribution volume of b.rGH, l.rGH and total rGH were measured in C, F, FR, D and D + l rats. Pituitary extracts, b.rGH and l.rGH were injected as a bolus, and the disappearance of each component was followed by RIA. In each group, the MCR of the big form was significantly lower than that of total rGH and l.rGH. Moreover, MCRs of total rGH, b.rGH and l.rGH in F and D rats were lower than that of the corresponding component in C animals. The decreased b.rGH MCR in each experimental group, and that of total rGH, b.rGH and l.rGH in F and D animals, is associated with a lower distribution space for the above cited components. Furthermore, the reduced receptor-binding activities for the larger form of rGH, and the loss of hepatic GH receptors in D rats, together with the possibility that b.rGH could be presumably excluded from the glomerular filtration and subsequent degradation, may be reasons for the longer persistence of b.rGH in plasma in each experimental group, and for prolonged permanence of l.rGH in plasma in F and D rats.