Refractoriness to growth hormone is associated with increased intracellular calcium in rat adipocytes.
In adipocytes that have been deprived of growth hormone (GH) for at least 3 hr, GH elicits a transient insulin-like response that is followed by a period of refractoriness to further insulin-like stimulation. Exposure of adipocytes to GH in the first hour of a 3-hr incubation prevents the appearance of insulin-like sensitivity. Intracellular Ca2+ concentration [( Ca2+]i) was measured in individual adipocytes that were loaded with fura-2 hexakis(acetoxymethyl) ester after preincubation in the presence (refractory) or absence (sensitive) of recombinant human GH at 100 ng/ml. Using a dual nitrogen laser imaging microscope with computer-assisted image processing to measure fluorescence changes, we observed that resting [Ca2+]i was 220 +/- 10 nM in refractory adipocytes and 110 +/- 6 nM in sensitive adipocytes (P less than 0.001). GH had no acute effect on [Ca2+]i in sensitive adipocytes but caused a sustained 3-fold increase in [Ca2+]i in refractory cells within 3 min (P less than 0.001). Insulin did not change [Ca2+]i in either sensitive or refractory adipocytes. In refractory cells treated with insulin and GH simultaneously, insulin completely blocked the rise in [Ca2+]i due to GH. Oxytocin elicited a prompt increase in [Ca2+]i followed by a quick return to resting levels in both sensitive and refractory cells. These findings indicate that basal [Ca2+]i is increased in refractory cells and that GH produces a sustained rise in [Ca2+]i only in refractory adipocytes. We suggest that the sustained increase in [Ca2+]i produced by GH in refractory cells prevents the expression of the insulin-like response.
Schwartz
Y
Y
University of Massachusetts Medical School, Department of Physiology, Worcester 01655.
Goodman
H M
HM
Yamaguchi
H
H
D000273:Adipose Tissue / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives; D013006:Growth Hormone / Q000494:pharmacology*; D007328:Insulin / Q000494:pharmacology; D008297:Male; D008856:Microscopy, Fluorescence / Q000295:instrumentation / Q000379:methods; D010121:Oxytocin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains
D005456:Fluorescent Dyes; D007328:Insulin; C049925:fura-2-am; D010121:Oxytocin; D013006:Growth Hormone; D002118:Calcium; D016257:Fura-2