Ja. Micol et Ml. Laorden, EFFECTS OF MU-AGONISTS, DELTA-AGONISTS AND KAPPA-AGONISTS ON ISOLATEDRIGHT ATRIA OF THE RAT, Neuropeptides, 26(6), 1994, pp. 365-370
The present study examined the effects of morphine, DAMGO, DPDPE and U
-50,488H on auricular rate on isolated right atria of the rat. All the
opioid agonists tested induced a decrease of auricular rate. The maxi
mal effect obtained with U-50,488H (75 +/- 8.3%) was significantly (p
< 0.001) higher than that obtained with morphine (12 +/- 2.7%), DAMGO
(8 +/- 0.6%) or DPDPE (11 +/- 1.8%). The inhibitory effects of U-50,48
8H were not antagonized by the presence of naloxone (10(-7) or 5 x 10(
-7) M) or MR-2266 (10(-7) or 5 x 10(-7) M). Moreover, U-50,488H did no
t change the auricular chronotropism in the presence of atropine (5 x
10(-7) M). In this case the maximal inhibitory effect was 79 +/- 6.7%,
similar to that obtained with the kappa-agonist alone (75 +/- 8.3%).
Propranolol (10(-8) or 5 x 10(-8) M) modified the inhibitory effect of
U-50,488H. The maximal effect obtained by the kappa-agonist in presen
ce of propranolol was 100 +/- 0 significantly (p < 0.01) higher than t
hat obtained with U-50,488H alone. These results demonstrated that the
depressant action of U-50,488H was not blocked in the presence of opi
oid receptor antagonists and probably does not involve opioid receptor
s. Furthermore, propranolol caused a dose-dependent potentiation of th
e effects of the kappa-agonist supporting the conclusion that it is no
t mediated by opioid receptors.