Studies on the Mechanism of Action of Glucagon in Strips of Rabbit Renal Artery

Abstract

The vasodilator effects of glucagon and adenosine cyclic 3′,5′‐monophosphate (cyclic AMP) were evaluated in strips of rabbit renal artery contracted with noradrenaline (NA) in the absence and presence of phosphodiesterase inhibitors or calcium (Ca2+) antagonists. The vascular relaxant effect of glucagon was markedly potentiated by various concentrations of four different phosphodiesterase inhibitors (papaverine, theophylline, 3‐isobutyl‐1‐methylxanthine (IBMX) and indomethacin), while that of cyclic AMP was potentiated by only two of them (papaverine and indomethacin) and inhibited by the others (theophylline and IBMX). Amongst the four phosphodiesterase inhibitors, IBMX (10 μg/ml) was found to produce the largest potentiation (e.g. the sensitivity increased by a factor of 10) of glucagon‐induced vascular relaxations (ED50 of glucagon in the presence of IBMX = 9.2 ± 1.0 ng/ml). Ca2+ antagonists such as verapamil and SKF 525A produced a dose‐dependent inhibition of the vasodilator action of glucagon. Verapamil (2.5 μg/ml) also antagonized cyclic AMP‐induced vascular relaxations. The vasodilator effect of verapamil was inhibited dose‐dependently by raising the concentration of extracellular Ca2+ from 0.05 to 0.2 g/l (or 1.25 to 5.0 mm) while those elicited by glucagon or cyclic AMP were not influenced, thus suggesting that the latter two drugs do not interfere with Ca2+ influx. Disodium edetate (Na2EDTA, 210 to 840 μg/l) produced a dose‐dependent vasodilator effect which was attributed to the facilitation of Ca2+ extrusion from the smooth muscle cells and/or Ca2+ binding to the cell membrane. The relaxation produced by Na2EDTA was significantly blocked by verapamil (10 μg/ml) or SKF 525A (10 μg/ml). The results were taken as an indication that glucagon produces at least a fraction of its vasodilator effect by promoting Ca2+ extrusion from the vascular smooth muscle cells and/or Ca2+ binding to or sequestration into intracellular sites, presumably via a cyclic AMP‐dependent mechanism.

Studies on the Mechanism of Action of Various Vasodilators

“…The tissues were suspended under a resting tension of 1 g in 40 ml organ baths containing a warm (37°C), oxygenated (95% 02: 5% C02) Krebs solution (Gagnon, Regoli & Rioux, 1978 other vasodilators are summarized in dose-response curve for histamine is shil by a factor of 4.3 + 0.5 in the presence and 9.6 + 0.7 in the presence of IBMX, parallel to the control curve. The extent is very similar to that reported previou gon (Gagnon et al, 1980) and similar e of papaverine and IBMX (Table 2).…”

“…Verapamil (5.1 and 51.0 x lo-7 M) also inhibits to varying extents the vasodilator effect of IBMX (Tables 1 and 2). (Gagnon et al, 1980) and other laboratories (for a review, see Fleckenstein, 1977) support this interpretation for drugs such as verapamil.…”

“…Several lines of evidence suggest that verapamil produces its vasodilator and cardiodepressant effects by interfering with the entry of Ca2+ into vascular smooth muscle or cardiac muscle cells (for a review, see Fleckenstein, 1977 (Gagnon et al, 1980). This unexpected finding led us to suggest that verapamil interferes with Ca2+ extrusion from and/or Ca2+ sequestration into the smooth muscle cells and through these mechanisms, inhibits drug-induced vasodilator effects.…”

“…The blockade of histamine-induced vascular relaxant effect by verapamil suggest that histamine, like glucagon and cyclic AMP (Gagnon et al, 1980) (Table 3). The mechanism by which exogenous adenosine provokes the relaxation of isolated vascular tissues is not known.…”

“…verapamil) (Gagnon, Regoli & Rioux, 1980 (Gagnon et al, 1980). The vasodilator effect of histamine was always measured in the presence of diphenhydramine (1.0 x 10-s M) in order to eliminate its HI-receptor-mediated contractile effect (Gagnon et al, 1978).…”