Neuronal deamination of endogenous and exogenous noradrenaline in the mesenteric artery of the spontaneously hypertensive rat

Transgenic mice expressing nerve growth factor in smooth muscle cells

Ectopic expression of nerve growth factor (NGF) in transgenic mice leads to site-specific sympathetic sprouting. Smooth muscle cells in the intestines, urinary bladder, and arteries have been shown to express NGF. To address whether enhanced NGF production among these different organ systems stimulates comparable patterns of sympathetic collateral growth, we generated transgenic mice that express NGF under the control of the smooth muscle alpha-actin promoter. In response to elevated levels of NGF protein in the colon, bladder, and arteries/arterioles, sympathetic axons displayed robust sprouting only in the colon and bladder. These data reveal that, unlike most other peripheral tissues, sympathetic efferents in adult mammalian arteries/arterioles do not undergo collateral growth in response to increased levels of smooth muscle-derived NGF.

Impaired function of alpha-2 adrenoceptors in smooth muscle of mesenteric arteries from spontaneously hypertensive rats

The alpha2-adrenoceptor function in mesenteric arteries of spontaneously hypertensive rats (SHR) was investigated by comparing membrane potential changes in response to adrenergic agonists in preparations from female SHR, Wistar-Kyoto (WKY) and normotensive Wistar rats (NWR). Resting membrane potential was found to be less negative in mesenteric arteries from SHR than in those from NWR and WKY. Apamin induced a decrease in the membrane potential of mesenteric artery rings without endothelium from NWR and WKY, but had no effects in those from SHR. Both UK 14,304 and adrenaline, in the presence of prazosin, induced a hyperpolarization that was significantly lower in de-endothelialized mesenteric rings from SHR than in those from NWR and WKY. In mesenteric rings with endothelium, however, similar hyperpolarization was observed in the three strains. In NWR mesenteric rings with endothelium the hyperpolarization induced by activation of alpha2-adrenoceptors was abolished by apamin, whereas in intact SHR mesenteric rings this hyperpolarization was slightly reduced by apamin and more efficiently reduced by Nomega-nitro-L-arginine. It is concluded that the activity of potassium channels coupled to alpha2-adrenoceptors is altered in the smooth muscle cells of SHR mesenteric arteries, contributing to their less negative membrane potential. On the other hand, the endothelial alpha2-receptors are functioning in mesenteric vessels from SHR and their stimulation induces a hyperpolarization mainly through the release of nitric oxide.

Angiotensin II receptors involved in the enhancement of noradrenergic transmission in the caudal artery of the spontaneously hypertensive rat

1. The effects of the AT1 receptor antagonist losartan and the AT2 receptor antagonist PD 123319, on actions of angiotensin II in isolated caudal arteries of spontaneously hypertensive (SH) and age-matched normotensive (Wistar-Kyoto) rats were compared. 2. Angiotensin II (0.1-3 microM) produced concentration-dependent increases in perfusion pressure in artery preparations from both SH and Wistar-Kyoto (WKY) rats, the maximal increase in the SH rat being significantly greater than the increase in WKY rats. The increase in perfusion pressure in preparations from both strains of rats was prevented by losartan (0.1 microM) and unaffected by PD 123319 (0.1 microM), indicating that the vasoconstrictor action of angiotensin II is subserved by AT1 receptors. 3. Angiotensin II (0.1-3 microM) produced concentration-dependent enhancement of both stimulation-induced (S-I) efflux of [3H]-noradrenaline and stimulation-evoked vasoconstrictor responses in isolated preparations of caudal artery from both SH and WKY rats, in which the noradrenergic transmitter stores had been labelled with [3H]-noradrenaline. The maximum enhancement of S-I efflux produced by angiotensin II (1 microM) was significantly greater in artery preparations from WKY rats than in preparations from SH rats, whereas the maximum enhancement of stimulation-evoked vasoconstrictor responses was greater in preparations from SH rats than in those from WKY rats. 4. In artery preparations from both WKY and SH rats, the AT1 angiotensin II receptor antagonist, losartan (0.01 and 0.1 microM), reduced or abolished the enhancement of both S-I efflux and vasoconstrictor responses by 1 microM angiotensin II. 5. The combination of 0.01 microM losartan and 0.1 microM angiotensin II enhanced both the S-I efflux and stimulation-evoked vasoconstrictor response in caudal artery preparations from WKY rats, whereas 0.1 microM angiotensin alone was ineffective. The AT2 receptor antagonist PD 123319 (0.01 and 0.1 microM) prevented the enhancement of both S-I efflux and stimulation-evoked vasoconstrictor responses by the combination of angiotensin II and losartan. 6. In contrast to findings in WKY preparations and those previously obtained for arteries from another normotensive strain (Sprague-Dawley), in artery preparations from SH rats there was no synergistic interaction between losartan and angiotensin II. Rather, combinations of 0.1 microM angiotensin II and PD 123319 (both 0.01 and 0.1 microM) enhanced S-I [3H]-noradrenaline efflux, whereas 0.1 microM angiotensin II alone was without effect. Moreover, losartan (0.1 microM) prevented the enhancement of S-I efflux by the combination of angiotensin II and PD 123319. 7. The present findings indicate that in the caudal artery of WKY and SH rats, and as previously found in Sprague-Dawley preparations, angiotensin II receptors similar to the AT1B subtype subserve enhancement of transmitter noradrenaline release. 8. As previously suggested for Sprague-Dawley caudal artery preparations, the synergistic prejunctional interaction of losartan and 0.1 microM angiotensin II in caudal artery preparations from WKY rats may be due to either the unmasking by losartan of a latent population of angiotensin II receptors subserving facilitation of transmitter noradrenaline release, or blockade by losartan of an inhibitory action of angiotensin II on transmitter release. 9. The synergistic interaction of PD 123319 and 0.1 microM angiotensin II in caudal arteries of SH rats may also be explained by either of the mechanisms proposed for the normotensive strains, but the involvement of different receptor subtypes would need to be postulated for each of the proposed mechanisms.

Changes in vascular endothelium and its function in systemic arterial hypertension

The various functions of arterial endothelium may be altered during pulmonary and arterial hypertension. Changes in the endothelium (or function) associated with hypertension are described. In both acute and chronic hypertension, permeability of the endothelium is enhanced. During the acute phase of hypertension, hyperplasia (cell replication) of the endothelium occurs while cell hypertrophy (enlarged cell size) and an increase in homocellular tight junctions are associated with sustained elevations of blood pressure. Endothelium may contribute to the increase in smooth muscle mass or cell number reported with various models of hypertension. Increased endothelial uptake or metabolism of norepinephrine and serotonin occurs during hypertension. The biotransformation of adenine nucleotides and various peptides by the endothelium is not altered by hypertension. Synthesis of prostacyclin is enhanced in the spontaneously hypertensive and Goldblatt hypertensive rat. Metabolism of prostaglandin E2, prostaglandin F2 alpha and prostacyclin by prostaglandin 15-hydroxydehydrogenase is impaired in the genetic models. Responses to endothelium-dependent vasodilators are impaired in acute and chronic models of hypertension. Production of relaxing factor by the endothelium is not inhibited, but rather the vascular smooth muscle fails to respond. Acute, severe hypertension potentiates the response to serotonin, presumably by attenuating the release or response to relaxing factor(s). In the aorta of the spontaneously hypertensive rat, the endothelium releases a constricting factor in response to acetylcholine. Pulmonary arterial endothelium (and other vessels) releases a vasoconstrictor that is blocked by inhibitors of cyclooxygenase. It is not clear whether this pressor factor is thromboxane A2. Cultured endothelial cells release a polypeptide that contracts arteries; however, any relation to hypertension is not known.

Enhanced 5-hydroxytryptamine release from vascular adrenergic nerves in spontaneously hypertensive rats

The release of 5-hydroxytryptamine from the vascular adrenergic nerve by periarterial nerve stimulation in spontaneously hypertensive rats (SHR) was compared with that in normotensive Wistar-Kyoto rats (WKY). The isolated mesenteric vascular bed was perfused at a constant flow rate of 5 ml/min. Vasoconstrictor responses to periarterial nerve stimulation (4, 8, 12, and 16 Hz for 30 seconds) and 5-hydroxytryptamine (1 microM), but not norepinephrine (1 nmol), were significantly greater in SHR than in WKY. After treatment with 5-hydroxytryptamine (1 microM) for 15 minutes, vasoconstrictor responses to periarterial nerve stimulation previously reduced by prazosin (50 nM) were restored and a frequency-dependent pressor response reappeared. However, 5-HT treatment did not significantly affect the pressor response to exogenously administered norepinephrine (1 nmol), which was previously inhibited by prazosin. The degree of the restoration in SHR was significantly greater than that in WKY at all frequencies used. The restoration of the pressor response to periarterial nerve stimulation after 5-hydroxytryptamine treatment did not occur in the presence of the selective 5-hydroxytryptamine2 receptor antagonists ketanserin (10 nM) or LY53857 (10 nM). In the perfused mesenteric vascular bed of both WKY and SHR prelabeled with [3H]5-hydroxytryptamine, periarterial nerve stimulation (4-16 Hz) evoked a frequency-dependent increase in tritium efflux that was abolished by Ca2+-free Krebs-Ringer solution or tetrodotoxin (100 nM) and treatment with 6-hydroxydopamine. The tritium efflux evoked by periarterial nerve stimulation was significantly greater in SHR than in WKY at all frequencies used. These results suggest that the release of 5-hydroxytryptamine from adrenergic nerve endings by periarterial nerve stimulation is enhanced in the mesenteric vascular bed of the SHR.