Relationship between morphine and isoprenaline-induced lipolysis. Quantitative aspects.

In order to estimate the relationship of the adipokinetic effect of morphine (MO) to adrenergic-mediated lipolysis the interaction between MO and isoprenaline (ISO) was studied in epididymal adipose tissue of rats and perirenal adipose tissue of rabbits. Two preparations of adipose tissue (AT)-trimmed tissue and fat cells (FC)-were used. Increment of non-esterified fatty acids (NEFA) release into the medium during 90 min of incubation was the measure of lipolysis. In rats, especially in FC, MO stimulated NEFA release up to 900% of basic values and also in rabbits the lipid-mobilizing action of MO was found as high as 320% of the non-stimulated level. The net increase due to MO expressed as % of maximum lipolysis induced by ISO was fluctuating between 30-50% except rat FC, where MO reached almost the level of ISO maximum effect. The calculated pD2 values of MO were in FC of both species 5.8, in trimmed AT 5.1 (rats) and 5.3 (rabbits). Different concentrations of MO (1-100 mumol/l) did not significantly alter the concentration-response adipokinetic curve of ISO. Naloxone in concentrations of 1 and 10 mumol/l did not exert any antagonistic action in MO-induced lipolysis. The MO effect is most probably not mediated via adrenergic receptors; the possibility of opiate receptor involvement has not been excluded. Pronounced species differences between rats and rabbits in sensitivity to the lipolytic morphine effect were observed.