Niels C. Berg Nyborg

AN INCREASED CALCIUM SENSITIVITY OF MESENTERIC RESISTANCE VESSELS IN YOUNG AND ADULT SPONTANEOUSLY HYPERTENSIVE RATS

1 We have measured the calcium sensitivity in response to noradrenaline stimulation and potassium depolarization of isolated segments of 100 to 200 μm mesenteric resistance vessels from spontaneously hypertensive (SHR) and control Wistar‐Kyoto (WKY) rats. The rats were either young (4 wk) or adult (4 months), that is of ages before or after the SHRs had developed elevated blood pressure. Experiments were performed under conditions in which the effect of noradrenaline uptake by, and the potassium‐induced noradrenaline release from, the nerve terminals in the vessel walls was eliminated. 2 The response of the SHR and WKY vessels to noradrenaline under conditions where only the extracellular calcium appeared to have been removed was similar. When subsequently stimulated maximally by noradrenaline, the calcium‐sensitivity of the SHR vessels (Ca‐ED50 ≃ 0.1 mm) was greater than that of the WKY vessels (Ca‐ED50 ≃ 0.2 mm). When depolarized by potassium, all vessels were less sensitive to calcium and there was little difference in the calcium sensitivity of SHR and WKY vessels in either age group (Ca‐ED50 ≃ 0.8 mm). 3 The results suggest that whereas the potassium (potential)‐dependent calcium permeability of the SHR smooth muscle cell membrane is normal, the noradrenaline‐induced calcium permeability is abnormally high. The presence of this abnormality in the vessels from the young SHRs suggests that it may be a factor involved in the aetiology of hypertension in the SHR.

Atrial natriuretic factor causes specific relaxation of rat renal arcuate arteries

1 We have investigated the effect of a synthetic ‘atrial natriuretic factor’ (ANF) on induced tone in rat isolated renal arcuate arteries (lumen diameter ca. 250 μm), and compared this with the effects of synthetic ANF on resistance vessels of similar size taken from the mesenteric, femoral, cerebral and coronary vasculature. 2 Synthetic ANF was found to cause relaxation of the renal vessels when these were sub‐maximally activated with K+, noradrenaline or 5‐hydroxytryptamine, but had no effect on the responses of the other vessels to these agonists. Synthetic ANF had a near maximal effect (65% relaxation) at 100 nM, with an IC50 of 7.9 nM. The relaxant effect of synthetic ANF on the renal vessels was fully maintained for at least 15 min. 3 Hydrallazine (100 μM) caused relaxation of renal vessels (47%) and coronary vessels (42%), but had no effect on the other vessel types. By contrast, sodium nitroprusside (1 μM) relaxed all vessel types. 4 The relaxant action of synthetic ANF on the renal vessels was seen in the presence of ouabain (1 mM), propranolol (1 μM), phentolamine (1 μM), atropine (1 μM) and felodipine (1 nM). 5 In the renal vessels, synthetic ANF had no effect on membrane potential, measured with intracellular electrodes, despite the simultaneously measured relaxation. 6 Synthetic ANF had no effect on the efflux of 22Na+ in either renal or mesenteric vessels. 7 The results demonstrate that synthetic ANF has a specific and prolonged relaxant effect on renal small arteries, and are consistent with this effect being mediated through specific receptors.

Role of the endothelium in acetylcholine-induced relaxation and spontaneous tone of bovine isolated retinal small arteries

Acetylcholine induced a variable concentration-dependent relaxation of bovine isolated retinal small arteries contracted with PGF2 alpha. The acetylcholine-mediated relaxation was linearly related to the sodium nitroprusside-induced relaxation, suggesting that the endothelium is well preserved in the vessels and that the variable effect of acetylcholine is due to variations in the soluble guanylate-cyclase enzyme activity in the smooth muscle. The vessels became desensitized to acetylcholine by repeated exposures. L-arginine and indomethacin did not abolish the desensitization. The vessels also became desensitized to the direct smooth muscle relaxing effect of sodium nitroprusside, indicating that desensitization to the endothelium-dependent relaxation by acetylcholine is related primarily to the vascular smooth muscle cells. Atropine, methylene blue and removal of endothelium abolished the acetylcholine-induced relaxation completely, whereas indomethacin had no inhibitory action on the acetylcholine-induced relaxation, suggesting that acetylcholine mediates release of EDRF (nitric oxide: NO) through stimulation of muscarinic receptors. Methylene blue contracted endothelium intact retinal arteries but a spontaneous tone was not present in endothelial denuded arteries. This may indicate a basal release of both a contractile factor, e.g. endothelin, and a relaxing factor. NO, form the retinal endothelium. The results demonstrate that endothelial-derived factors may participate in normal as well as pathophysiological regulation of retinal vascular smooth muscle tone.

Active, passive and myogenic characteristics of isolated rat intramural coronary resistance arteries

We have investigated the active, passive and myogenic tension-internal circumference relations of rat intramural coronary and, as controls, mesenteric small arteries (internal diameter ca. 200 μm) using an isometric myograph. The active tensions of the vessels (when fully activated with 30 μM serotonin in K-saline) reached a maximum (2.54 N/m, coronary; 3.39 N/m, mesenteric) at an internal circumference, L0, where the passive tensions (measured in Ca-free solution) were 0.80 N/m (coronary) and 0.74 N/m (mesenteric). Below 0.8 L0 and above 1.2 L0 the active tensions fell linearly, the zero tension intercepts being 0.37 L0 and 1.74 L0 (coronary) and 0.40 L0 and 1.72 L0 (mesenteric). The passive wall tensions of the vessels rose exponentially as a function of internal circumference, the wall tension at 1.5 L0 being 10.0 N/m (coronary) and 8.5 N/m (mesenteric). In normal physiological salt solution, the coronary vessels had a Ca2+ dependent myogenic tone which was also dependent on the internal circumference. Maximum myogenic tone (0.54 N/m) was obtained at 1.18 L0. The mesenteric vessels had no such myogenic tone. Histological examination showed that the media/lumen ratios of both vessel types were the same, and that the smooth muscle content of the media was greater in the coronary (81%) than in the mesenteric (72%) vessels. The smaller active tension of the coronary vessels could not therefore be ascribed to a reduced smooth muscle content, but possibly in part to an observed heterogeneous arrangement of the smooth muscle cells in the coronary vessels.

Heterogeneous involvement of endothelium in calcitonin gene-related peptide-induced relaxation in coronary arteries from rat

1 The effects of rat‐ and human‐CGRP and capsaicin were studied in isolated rings of rat proximal epicardial (PC) and distal intramyocardial (DC) coronary arteries. 2 The relaxing effect of rat‐CGRP was dependent on the level of vessel tone induced by prostaglandin F2α (PGF2α) in PC but not in DC arteries. Submaximally contracted DC and PC arteries were more sensitive to rat‐ than human‐CGRP. There was no difference in sensitivity to rat‐ and human‐CGRP between PC and DC arteries. 3 Substance P elicited a small relaxation only in 4 of the 6 PC arteries tested. PC and DC arteries were concentration‐dependently relaxed by capsaicin. The relaxation was partly inhibited by ruthenium red, thus suggesting that capsaicin causes specific release of CGRP from sensory nerve endings in rat coronary arteries. 4 The relaxant effect of rat‐CGRP was antagonized by endothelium removal and indomethacin but not methylene blue in endothelium‐intact PC arteries. The relaxation in DC arteries was not affected by any of these treatments, indicating a heterogeneous involvement of the endothelium in CGRP‐mediated coronary vasodilatation and the release of a cyclo‐oxygenase product in PC arteries in rats. 5 Glibenclamide had no inhibitory effect on the CGRP‐induced relaxation of PC and DC arteries, thus excluding the involvement of glibenclamide‐sensitive K+‐channels in the mechanism of action of CGRP in rat coronary arteries.

Contractile and relaxing effects of arachidonic acid derivatives on isolated bovine retinal resistance arteries

The reactivity of isolated ring segments (i.d. ca. 200 microns) of calf retinal resistance arteries to arachidonic acid derivatives were studied in vitro. Prostaglandin F2 alpha (PGF2 alpha) prostaglandin E2 (PGE2), and a thromboxane A2 analogue, 9,11-dideoxy-9 alpha,11 alpha-epoxymethane-prostaglandin F2 alpha, (TX2 alpha), induced a concentration dependent contraction with the order of potency: Tx2 alpha greater than PGE2 greater than PGF2 alpha with pD2-values (-log[EC50(M)]) of 6.88, 6.19 and 5.20, respectively. The maximal active tension development of the vessels induced by Tx2 alpha, PGE2 and PGF2 alpha, were 1.57 N m(-1), 1.05 N m(-1) and 1.19 N m(-1), corresponding to 94%, 57% and 67% of Emax, respectively. The active pressure development, and estimate of the maximum pressure which the vessels may be able to contract against in vivo, was 18 kPa, 12 kPa and 14 kPa (138, 89 and 110 mmHg) for Tx2 alpha, PGE2 and PGF2 alpha, respectively. Prostaglandin I2 (prostacyclin, PGI2) induced a concentration-dependent relaxation of retinal resistance arteries precontracted with 10(-5) M PGF2 alpha. The maximal relaxation induced by PGI2 amounted to 65% of the control vessel response induced by PGF2 alpha (1.27 N m(-1] with a -log[EC50(M)] value of 6.51. The results indicate that arachidonic acid derivatives are potent and effective agents capable of regulating bovine retinal resistance artery smooth muscle tension in vitro.

Evidence that the Increased Calcium Sensitivity of Resistance Vessels in Spontaneously Hypertensive Rats is an Intrinsic Defect of Their Vascular Smooth Muscle

We have investigated media thickness and noradrenaline- and calcium-sensitivity of isolated mesenteric resistance vessels from spontaneously hypertensive (SHR) and control Wistar-Kyoto (WKY) rats, which had been treated in various ways. Vessels from untreated SHRs had an increased media thickness and increased noradrenaline- and calcium-sensitivity. Anti-hypertensive treatment of SHRs and WKYs with Felodipine reduced blood pressure, eliminated the difference in vessel media thickness, but did not reduce vessel noradrenaline- or calcium-sensitivity. Chemical sympathectomy of SHRs and WKYs at birth likewise resulted in reduced blood pressure, but did not eliminate the difference in vessel media thickness although again vessel noradrenaline- and calcium-sensitivity remained unchanged. Induction of renal hypertension in WKYs caused an increase in blood pressure and increase in vessel media thickness, but did not affect vessel noradrenaline-sensitivity. The results suggest that while vessel media thickness is influenced by blood pressure, the sensitivity differences of SHR resistance vessels are an intrinsic defect of their vascular smooth muscle, independent both of blood pressure and neurogenic influences.

Active wall tension — Length curve and morphology of isolated bovine retinal small arteries: Important feature for pharmacodynamic studies

The active and passive wall tension--internal circumference (diameter) relations of isolated ring segments of bovine retinal small arteries (i.d. ca. 200 microns) were studied in vitro using an isometric myograph. The active tension of the vessels (when fully activated with 10(-5) M 5-HT and 10(-5) M PGF2 alpha in high K(+)-saline) reached a maximum (1.2 N m-1) at an internal circumference, L0, where the passive tension (measured in Ca2(+)-free solution) were 0.89 N m-1. Below and above L0 the active tension fell linearly, the zero tension intercepts being 0.48 L0 and 1.61 L0. The passive wall tension rose exponentially as a function of the internal circumference (diameter) and was 43 times higher at 1.4 L0 than at L0. A small transient spontaneous contraction was observed after rapid stretch of the arteries. Papaverine and Ca2(+)-free solution did not change the wall tension of vessels stretched to L0, indicating absence of stable spontaneous myogenic tone in the vessels. Histological examination showed that the retinal artery media on average had three smooth muscle cell layers which in total were 12 microns thick. The media/lumen ratio was 7%. Smooth muscle cell volume was 1228 micron3, constituting about 72% of the media volume. The active wall tension--internal circumference relation and morphology of bovine retinal small arteries is generally equal to that of arteries of similar size from other species, but active force generation of the retinal vascular smooth muscle cells is considerably lower.

Adrenergic responses in isolated bovine retinal resistance arteries

SummaryThe reactivity of bovine retinal resistance arteries (i.d. ca. 200 μm) to selective alpha1-, beta- and combined alpha- and beta-adrenoceptor stimulation was studiedin vitro using phenylephrine, isoproterenol and norepinephrine, respectively. The results indicate that only functional alpha1-adrenoceptors, possibly in low numbers, are present in these vessels.

Calcium antagonist-induced relaxation of the prostaglandin-F2α response of isolated calf retinal resistance arteries

The relaxing effect of nitrendipine and D600 on isolated ring segments of calf retinal resistance arteries (i.d. ca. 200 microns) contracted with prostaglandin-F2 alpha (PGF2 alpha) or high potassium solution was studied. The vessel response to both PGF2 alpha and potassium stimulation was dependent on extracellular calcium. Removal of extracellular calcium reduced the vessel response to PGF2 alpha and potassium by 85% and 98%, respectively (P less than 0.01). Both calcium antagonists induced a concentration dependent relaxation of PGF2 alpha pre-contracted vessels with = -log[IC50(M)]-values of 8.01 and 7.13 for nitrendipine and D600 (P less than 0.05), respectively. The calcium antagonists were equieffective in relaxing the vessels amounting to 33% for nitrendipine at 10(-6) M and 37% for D600 at 10(-5) M. Further analysis of the data revealed that the nitrendipine induced relaxation of the PGF2 alpha response was linearly correlated with the internal lumen diameter of the vessels. No correlation was found for the D600 induced relaxation. At the highest concentrations of nitrendipine, 10(-6) M, and D600, 10(-5) M, the potassium induced response was reduced by 94 and 75%, respectively. The results show that calcium antagonists only partially relax retinal resistance arteries contracted with PGF2 alpha although the vessels are dependent on extracellular calcium for active force generation. Calcium must therefore be activated by PGF2 alpha through other pathways than those blocked by calcium antagonists.