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.

The vascular relaxant effects of histamine, adenosine, isoprenaline nitroglycerine, papaverine and 3‐isobutyl‐1‐methylxanthine (IBMX) were assessed individually, in strips of rabbit renal artery moderately contracted with noradrenaline (NA) in the absence or presence of phosphodiesterase inhibitors (papaverine and IBMX) or verapamil, a Ca2+ antagonist. The vasodilator effect of histamine was potentiated by papaverine (6.1 × 10−7 m) and IBMX (4.4 × 10−5 m) but inhibited dose‐dependently by verapamil (5.1 and 51.0 × 10−7 m). Adenosine‐induced vascular relaxations were greatly increased in the presence of papaverine (6.1 × 10−7 m) but significantly reduced in the presence of IBMX (4.4 × 10−5 m) or verapamil (5.1 and 51.0 × 10−7 m). The vasodilatation produced by isoprenaline was increased in the presence of IBMX (4.4 × 10−5 m) or papaverine (6.1 × 10−7 m), but inhibited by verapamil (5.1 and 51.0 × 10−7 m). The vascular relaxant effects of nitroglycerine and papaverine were inhibited in the presence of IBMX (4.4 × −5 m) or verapamil (5.1 and 51.0 × 10−7 m). Papaverine (6.1 × 10−7 m) also antagonized nitroglycerine‐induced vascular relaxation. The vasodilator effect of IBMX was greatly reduced in the presence of papaverine (6.1 × 10−7 m) or verapamil (5.1 and 51.0 × 10−7 m). The vascular relaxant effect of verapamil was reduced proportionally by raising the extracellular Ca2+ concentration from 1.25 to 5.0 mm while those elicited by histamine, adenosine, isoprenaline, nitroglycerine, papaverine and IBMX were not modified by this procedure. These results were taken as an indication that several vasodilators (e.g. histamine, adenosine, isoprenaline, nitroglycerine, papaverine and IBMX), but not a Ca2+ antagonist such as verapamil, produce a fraction of their vasodilator effects by promoting Ca2+ extrusion from and/or Ca2+ sequestration into the vascular smooth muscle cells, via a cyclic adenosine 3′,5′‐monophosphate‐dependent mechanism.

1 The relaxant action of glucagon has been studied in strips of rabbit renal arteries partially contracted by a low concentration (1 ng/ml) of noradrenaline. 2 The preparation was relaxed in a dose-dependent manner by concentrations of glucagon varying between 25 ng/ml and 420 ng/ml. 3 The relaxant effect of glucagon (0.1 1tg/ml -ED60) on this preparation was not affected by propranolol (5.0 jg/ml), cimetidine (10 gg/ml), diphenhydramine (10 jg/ml), indomethacin (5.0 j.g/ml), phentolamine (1.2 pg/ml), atropine (10 tLg/ml) and 8-Leu-AT,, (1.0 gg/ml) but was slightly potentiated by Des-Arg9 Leu-OMe8-Bk (25 gg/ml) and indomethacin (50 jig/ml). 4 The dose-response curve to glucagon remained parallel in the presence of papaverine (2.5 gg/ml) but was shifted to the left by a factor of 2.5 to 2.8. Theophylline (250 jtg/ml) also potentiated the vascular relaxation induced by glucagon.5 Insulin (10 glg/ml) did not influence the relaxant effect of glucagon.6 The removal of the N-terminal amino acid (His) of glucagon reduced by 89% the biological activity of this fragment on the vascular preparation. The removal of the C-terminal amino acids Met-27, Asn-28 and Thr-29 of glucagon resulted in a fragment which was inactive either as an agonist or as an antagonist when tested at concentrations as high as 925 ng/ml. 7 It is concluded that the relaxation of partially contracted strips of rabbit renal arteries by glucagon constitutes a simple, sensitive, relatively specific and reliable bioassay which may be useful for the determination of glucagon in biological materials and for structure-activity relationship studies with this hormone.

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