ALTERED GROWTH-HORMONE (GH) SECRETION IN-VIVO AND IN-VITRO IN THE DIABETES-PRONE BB WORCESTER RAT/

Diminished concentrations of growth hormone (GH) have been observed in the male BB/Wor rat with diabetes mellitus (DM). The precise mechanis m(s) responsible for the altered GH levels is not entirely understood. We have therefore employed independent techniques to investigate pote ntial alterations in: 1) the peripheral metabolism of the hormone; 2) GH release by somatotropes; and 3) hypothalamic regulation of GH secre tion. An extra group of insulin-untreated animals was included for the studies of acute DM. The results demonstrate diminished circulating m ean concentrations of GH (35 +/- 4 vs. 16 +/- 4 mu g/l; mean +/- SEM; control vs, animal with DM; P = 0.006) due to impaired GH secretion. I n particular, there was a decrease in the mass of GH secreted per burs t (230 +/- 22 vs. 136) 34 +/- mu g/l; P = 0.04) and in the GH secretor y rate (24 +/- 4 vs. 9 +/- 3 mu g/l/min; P < 0.01). No differences in the secretory burst frequency, (5.3 +/- 0.3 vs. 5.2 +/- 0.5 #/8-h; P = 0.68), secretory half-duration (10 +/- 2 vs. 17 +/- 2 min; P = 0.09), or serum GH half-life (8 +/- vs. 6 +/- 1 min; P = 0.13) were observed . In vitro studies of acutely dispersed somatotropes obtained from rat s with DM demonstrated increased sensitivity to GHRH (1 nM), as detect ed by a greater mean hemolytic plaque area following exposure to an EC ,, dose of the secretagogue (14.3 +/- 3.3 vs. 17.4 +/- 3.5 mu m(2) x 1 0(3); P = 0.049), and diminished sensitivity to SRIH (1 nM) inhibition of GH release following exposure to an EC,, dose of the secretagogue (10.0 +/- 1.2 vs. 14.9 +/- 2.3 mu m(2) x 10(3); P = 0.026). The number of the pituitary cells (18.0 +/- 2.8 vs. 15.3 +/- 1.0 x 10(5) cells; P = 0.38) as well as the number of somatotropes (7.3 +/- 1.4 vs. 7.6 /- 0.9 x 10(5) cells; P = 0.87) were indistinguishable between experim ental groups. Hypothalamic gene transcript levels for GH-releasing hor mone (GHRH) and somatotropin release-inhibiting hormone (SRIH) were ev aluated by in situ hybridization histochemistry to assess cellular syn thetic activity. Insulin-treated and non-treated animals with acute DM (2 days duration) demonstrated a decrease in GHRH mRNA levels as asse ssed by densitometric analysis of (8.0 +/- 0.5 vs. 6.1 +/- 0.7 vs. 6.0 +/- 0.5 mu m(2) x 10(4), control vs. insulin-treated vs. non-treated, P <0.05) and grain counting measurements (110 +/- 7 vs. 66 +/- 5 vs. 70 +/- 6 mu m(2), P <0.05). The number of neurons expressing the GHRH gene was indistinguishable among the groups. No changes in GHRH gene t ranscript levels were observed in animals with chronic DM (>2-21 days duration). No differences in preproSRIH mRNA levels were evident in ra ts with acute and/or chronic DM. We conclude that: 1) diminished GH co ncentrations in the BB/Wor rat with DM are due to diminished rate and mass of GH released per secretory episode without any alteration in th e number or duration of secretory events, or in the estimated metaboli c clearance rate of GH; 2) in vitro evidence of enhanced sensitivity o f somatotropes from rats with DM to GHRH was found suggesting attenuat ed GHRH release in vivo. Conversely, somatotropes from animals with DM demonstrated decreased sensitivity to SRM suggesting increased SRIH e xposure in vivo; 3) GHRH gene transcript levels were diminished acutel y in, the BB/Wor rat following the onset of DM, but were invariant bey ond 2 and up to 21 days after the development of DM. The altered GHRH mRNA levels may be, at least initially, causally related to the dampen ed pituitary GH secretion and the consequent lowered GH concentrations . Changes in hypothalamic SRIH gene transcript levels were not demonst rable in the diabetic animals.