Quantitative analysis of the lipid-mobilizing effects of corticotropin, beta 1-24--corticotropin (tetracosactide) and isoprenaline in older rats and rabbits.

The lipid mobilizing effects of corticotropin (ACTH), beta 1-24-corticotropin-tetracosactide (TCA) and isoprenaline (ISO) were studied in parallel experiments in older rabbits (six months, weight 3.5 kg) and rats (age 75-85 days, weight 275 g) in vitro. The results were analysed quantitatively. Compared with the effect in younger rats the lipid mobilizing action of ISO in older animals does not differ. The same applies to the very low ISO lipolytic potency in rabbits. The ACTH intrinsic activity and affinity decrease in aging rabbits, but not in aging rats. At the concentration 1 x 10(-9) mol/l of TCA 35% of its maximum effect is reached in rabbits and almost the maximum in rats. In comparison with the maximum response to ACTH the maximum of TCA is two times higher in rabbits whereas in rats it reaches only 80% of the ACTH maximum. The order of the intrinsic activities is in older rats and rabbits different: ISO much greater than ACTH greater than TCA (1.00:0.81:0.64) in rats and TCA greater than ACTH greater than ISO (1.00:0.53:0.31) in rabbits. The affinity relations TCA greater than ISO greater than ACTH are identical in both species with the ratios of ACTH:ISO:TCA = 1:50:1740 in rats and 1:34:915 in rabbits. TCA exerts the strongest lipolytic potency in rats which is 5.5 times greater than in rabbits. On the contrary the intrinsic activity of TCA in rabbits is the highest. In older rabbits the ACTH adipokinetic intrinsic activity and affinity decrease faster than those in ISO. The species differences especially in relation to ISO also remain preserved in older animals and this applies to the TCA lipolytic effects, which in turn do not seem to change with aging as much as the effects of ACTH. When evaluating the species differences or the sensitivity changes connected with aging in the action of lipolytic drugs, the intrinsic activity and affinity should be considered separately.