Presynaptic alpha 2-adrenoceptor mediated regulation of norepinephrine release in perfused mesenteric vasculatures in young and adult spontaneously hypertensive rats

The present study was designed to evaluate the role of the presynaptic alpha 2-adrenoceptor in the pathogenesis of hypertension. Norepinephrine overflow during sympathetic nerve stimulation and its changes by presynaptic alpha 2-adrenoceptor inhibition were examined in the perfused mesenteric vasculatures of young and adult spontaneously hypertensive rats (SHR) compared with age-matched Wistar Kyoto rats (WKY). Electrical sympathetic nerve stimulation caused significantly greater overflow of endogenous norepinephrine from the adrenergic nerve terminals in young SHR than in age-matched WKY. Yohimbine, an alpha 2-adrenoceptor blocking agent, facilitated norepinephrine overflow from the adrenergic nerve terminals. The effects of yohimbine on norepinephrine overflow and pressor responses to electrical nerve stimulation were less in young SHR than in age-matched WKY. Norepinephrine overflow in adult SHR was similar to that in adult WKY, and differences in the effect of yohimbine on norepinephrine overflow between SHR and WKY were not marked at this chronic stage. These results suggest that enhanced norepinephrine overflow in the mesenteric vasculatures can be observed only in young SHR; this may be due in part to an impaired negative feed-back mechanism on the nerve terminals by presynaptic alpha 2-adrenoceptors.     

Diminished Prostaglandin-Mediated Inhibition of Norepinephrine Release from the Sympathetic Nerve Endings in Spontaneously Hypertensive Rats

This study was performed to investigate prostaglandin(PG)-mediated regulation of norepinephrine release from the sympathetic nerve endings in spontaneously hypertensive rats(SHR). The effects of PGE₂ on the pressor responses and norepinephrine overflow during the periarterial nerve stimulation were examined in the perfused mesenteric vascular beds from SHR and age-matched Wistar Kyoto rats(WKY). In preliminary studies using normotensive Wistar rats, PGE₂ inhibited the overflow of norepinephrine in response to the electrical nerve stimulation. On the contrary, when indomethacin was infused into the preparations, the norepinephrine overflow induced by the nerve stimulation was increased. In SHR, the neurogenic vasoconstriction and norepinephrine overflow during the nerve stimulation were significantly greater than in WKY. PGE₂ in the medium inhibited the pressor responses and norepinephrine overflow during the nerve stimulation dose-dependently in WKY, while the inhibitory effects of PGE₂ in SHR were blunted. These results suggest that PGE₂ is an important hormone in regulating sympathetic vascular tone and that presynaptic inhibition of vascular adrenergic transmission by PGE₂ is attenuated in SHR.     

The Role of Calmodulin in Neurotransmitter Release and Vascular Responsiveness in Spontaneously Hypertensive Rats

ABSTRACT

This study was designed to investigate the role of calmodulin in adrenergic transmission in hypertension. In perfused mesenteric vasculature: from spontaneously hypertensive rats(SHR, 7-9 weeks of age) and age-matched Wistar Kyoto rats(WKY), the effects of a specific calmodulin antagonist(W-7) on norepinephrine overflow and vascular responsiveness to endogenous and exogenous norepinephrine were examined.

The vasoconstrictor responses to electrical nerve stimulation and exogenous norepinephrine as well as norepinephrine overflow during electrical nerve stimulation were significantly enhanced in SHR compared with those in age-matched WKY. The calmodulin antagonist, W-7, reduced not only vasoconstrictor responses but also norepinephrine overflow during nerve stimulation. These inhibitory effects of W-7 were significantly greater in SHR than in WKY.

The results demonstrate that norepinephrine overflow from the sympathetic nerve endings and vascular responsiveness were increased in SHR. The marked reduction in norepinephrine overflow and pressor responses by W-7 might suggest the greater calmodulin-dependent adrenergic transmission in this model of hypertension.     

Effects of ouabain on adrenergic neurotransmission in spontaneously hypertensive rats

The purpose of the present study is to determine the role of Na+,K+-ATPase in adrenergic neurotransmission of hypertension. Isolated perfused mesenteric vasculatures were prepared from spontaneously hypertensive rats (SHR, Okamoto and Aoki, 7-10 weeks old) and age-matched normotensive Wistar Kyoto rats (WKY). The effects of ouabain, a Na+,K+-ATPase inhibitor, on the norepinephrine overflow from the sympathetic nerve endings were examined. Norepinephrine overflow from the nerve endings as well as pressor responses during electrical nerve stimulation were significantly greater in SHR than in WKY. Ouabain increased the norepinephrine overflow evoked by electrical nerve stimulation, even in the presence of an uptake-blocker of norepinephrine. Further, the facilitatory effect of ouabain on stimulation-induced norepinephrine overflow was more prominent in SHR than in WKY. These results suggested that ouabain-sensitive Na+,K+-ATPase on sympathetic nerve terminals could have an important role in the regulation of neurotransmitter release, and that its activity might be enhanced in SHR compared with WKY.     

Inhibitory action on alpha-human atrial natriuretic polypeptide on vascular adrenergic neurotransmission is attenuated in spontaneously hypertensive rats

The purpose of the present study is twofold, firstly to investigate the effects of alpha-human atrial natriuretic polypeptide (alpha-hANP) on norepinephrine overflow from sympathetic nerve endings, and secondly to compare vascular responsiveness in perfused mesenteric preparations in spontaneously hypertensive rats (SHR, Okamoto and Aoki, 7-9 weeks old) and a cohort of Wistar Kyoto rats (WKY). In preliminary studies using normotensive Wistar rats, the pressor responses to electrical nerve stimulation or exogenous norepinephrine application were inhibited by alpha-hANP. Norepinephrine overflow was also suppressed by alpha-hANP, during nerve stimulation. The pressor responses and norepinephrine overflow during nerve stimulation were significantly greater in SHR than in WKY rats. The inhibitory effect of alpha-hANP on these responses was reduced in SHR. These results indicate that alpha-hANP could affect both pre- and post-synaptic sites of the resistance vessels. Further, the reduced inhibition of pressor responses and norepinephrine overflow by alpha-hANP in SHR suggests an insufficient regulation of adrenergic transmission by alpha-hANP in hypertension.     

The role of epinephrine as a neuromodulator in spontaneously hypertensive rats

The present study was carried out to elucidate the role of epinephrine as a neuromodulator in hypertension. The effects of epinephrine on norepinephrine release from the sympathetic nerve endings were examined in isolated perfused mesenteric arteries of spontaneously hypertensive rats (SHR) and age-matched Wistar Kyoto rats (WKY). Norepinephrine overflow during electrical nerve stimulation (5, 15 Hz) was significantly greater in SHR than in WKY. Low concentration of exogenous epinephrine (5.5 X 10(-9) M) potentiated norepinephrine overflow during nerve stimulation in SHR, and this (at 15 Hz stimulation) was antagonized by propranolol (5.0 x 10(-7) M), whereas, the overflow in WKY was reduced by the same concentration of epinephrine. A higher concentration of epinephrine (1.4 x 10(-8) M) decreased norepinephrine overflow in both SHR and WKY, and this change (at 15 Hz stimulation) was antagonized by yohimbine (1.0 x 10(-7) M). Further, magnitudes of the suppressions were smaller in SHR than in WKY. These results suggest that altered modulations of norepinephrine release by epinephrine through presynaptic beta- and alpha 2-adrenoceptors might induce increased sympathetic nerve activity in SHR.     

Norepinephrine overflow in perfused mesenteric arteries of spontaneously hypertensive rats

We examined the overflow of endogenous norepinephrine with electrical stimulation, the associated pressor response, and rate of initial neuronal uptake of [3H]norepinephrine in perfused mesenteric arteries of 7- and 13-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. The tissues of two rats, a spontaneously hypertensive and a WKY control rat, were simultaneously processed and subjected to the same electrical stimulation. Both absolute and fractional overflow of endogenous norepinephrine during periarterial nerve stimulation (5 and 10 Hz for 1 minute) in the tissue of 7-week-old SHR was significantly greater whereas overflow of 13-week-old SHR was equivalent as compared with that of the age-matched WKY rats. The tissue content of norepinephrine was 20-25% higher in SHR of both ages. There was significantly enhanced [3H]norepinephrine uptake in the tissues of young SHR, but no difference was observed in the older SHR. The pressor response to periarterial nerve stimulation was significantly enhanced in 7-week-old SHR and much more so at the older age as compared with the WKY control rats. Exogenous norepinephrine dose-response curves in the tissues of 7-week-old SHR exhibited a parallel leftward shift, characteristic of a change in sensitivity, whereas that of 13-week-old SHR showed a much steeper slope as compared with the respective WKY control rats. This finding suggests that in addition to smooth muscle supersensitivity, structural alterations had occurred in vasculature of 13-week-old SHR. These data indicate that in SHR both the exocytotic release of norepinephrine and the responsiveness of the vascular smooth muscle cells are enhanced in the developmental stage of hypertension whereas smooth muscle supersensitivity to norepinephrine and nonspecific structural alterations primarily contribute to the maintenance of hypertension at 13 weeks of age.     

Enhanced neuroinhibitory effect of diltiazem in blood vessels of spontaneously hypertensive rats

This study investigated the effects of diltiazem (a Ca2(+)-entry blocker) on neuromuscular junctions of blood vessels in hypertension. In isolated perfused mesenteric vasculatures prepared from spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY), the effects of diltiazem on norepinephrine release from vascular adrenergic neurons and pressor responses were examined. The influences of extracellular Ca2(+)-reduction on these responses were also studied. Stimulation evoked pressor responses and norepinephrine release were significantly greater in the mesenteric vasculatures of SHR than in those of WKY. Diltiazem inhibited both pressor responses and norepinephrine release during electrical nerve stimulation in a dose-dependent manner. The suppression of these responses was more pronounced in SHR than in WKY. Reduction of extracellular Ca2(+)-concentration also decreased the responses in SHR and WKY, and the inhibitory degree was significantly greater in SHR than in WKY. These results demonstrate that diltiazem affected the presynaptic site of the mesenteric vasculatures and decreased the stimulation-evoked norepinephrine release from vascular adrenergic neurons with a concomitant reduction of pressor responses of the preparation. Furthermore, the marked inhibition of pressor responses and norepinephrine release by diltiazem or Ca2(+)-depletion in SHR may suggest the increased Ca2(+)-dependency in vascular neurotransmission in this model of hypertension.     

Effects of Captopril on Neurosecretion and Vascular Responsiveness in Hypertension

The purpose of the present study was to investigate the mechanisms of hypotensive action of an angiotensin converting enzyme inhibitor(captopril) in hypertension. In perfused mesenteric vasculatures from spontaneously hypertensive rats(SHR) and normotensive Wistar Kyoto rats(WKY), the effects of captopril on the vascular responsiveness and norepinephrine overflow from the adrenergic nerve endings were examined

The vasoconstrictor responses and norepinephrine overflow during the electrical nerve stimulation was significantly enhanced in SHR compared with WKY. Captopril reduced not only vasoconstrictor responses but also norepinephrine overflow during the nerve stimulation in a dose-dependent fashion. The suppressions of these responses by captopril were significantly greater in SHR than in WKY

These results demonstrate that captopril could affect the presynaptic site of the resistance vessels and cause a decrease in electrically-stimulated norepinephrine overflow from the adrenergic nerve endings. The marked reduction of the pressor responses and norepinephrine overflow to nerve stimulation by captopril in the SHR suggests that the renin-angiotensin system in the vascular beds is enhanced in this model of hypertension.     

The role of dopamine in the regulation of neurotransmitter release in spontaneously hypertensive rats

The purpose of the present study was to analyze the influence of dopamine on norepinephrine release in resistance vessels in spontaneously hypertensive rats (SHR). Perfused mesenteric vasculature preparations from spontaneously hypertensive rats (7-10 weeks old) and age-matched normotensive Wistar Kyoto rats (WKY) were used to compare the effects of dopamine on both pressor responses and norepinephrine release. Both responses to electrical nerve stimulation were significantly greater in SHR than in WKY rats. Dopamine reduced these responses in a dose-dependent manner in WKY. However, this suppression of responses to electrical stimulation was attenuated in SHR. These results suggest that the enhanced adrenergic transmission in SHR may partly reflect impaired dopamine-mediated inhibition of nerve terminals, which would contribute to the pathogenesis of hypertension.     

Inhibitory actions of captopril on norepinephrine release from adrenergic nerve endings in spontaneously hypertensive rats

The purpose of the present study was to investigate the mechanisms of the hypotensive actions of an angiotensin converting enzyme inhibitor of the hypotensive actions of an angiotensin converting enzyme inhibitor (captopril) in hypertension. In perfused mesenteric vasculatures from spontaneously hypertensive rats (SHR, Okamoto and Aoki strain, 10-13 weeks of age) and age-matched normotensive Wistar Kyoto rats (WKY), the effects of captopril on vascular responsiveness and norepinephrine release from the adrenergic nerve endings were examined. The vasoconstrictor responses and norepinephrine release during the electrical nerve stimulation were significantly enhanced in SHR compared to those in age-matched WKY. Captopril reduced both vasoconstrictor responses and norepinephrine release during the electrical nerve stimulation, dose-dependently. These inhibitory effects of captopril were significantly greater in SHR than in WKY. The results demonstrate that captopril affects presynaptic sites on the resistance vessels and causes a decrease in electrically-stimulated norepinephrine release from the adrenergic nerve endings. The marked reduction of both pressor responses and norepinephrine release by captopril in SHR suggests an enhanced renin-angiotensin system in the vascular walls of hypertension.     

Neurotransmitter Release,Vascular Responsiveness and Their Suppression by Ca-Antagonist in Perfused Mesenteric Vasculature of Doca-Salt Hypertensive Rats

To investigate the role of norepinephrine release from the sympathetic nerve endings and vascular responsiveness in the pathogenesis of hypertension, the perfused mesenteric preparations were used in DOCA-salt hypertensive rats (acute phase: 10 days after operation, chronic phase: 7–8 weeks).

In addition, the effects of a Ca-antagonist (verapamil) on the norepinephrine release and vascular responsiveness were also examined.

Vasoconstrictor responses to the electrical nerve stimulation were significantly greater in DOCA-salt hypertension in the chronic phase than the age-matched normotensive controls. The pressor responses to exogenous norepinephrine were significantly enhanced in DOCA-salt hypertension both in acute and chronic phases.

Endogenous norepinephrine overflow from the sympathetic nerve endings during the electrical nerve stimulation was enhanced in the chronic phase of DOCA-salt hypertension, but not in the acute phase, compared with the age-matched normotensive controls.

After infusion of verapamil, the pressor responses and norepinephrine overflow by the electrical nerve stimulation were significantly inhibited, and the suppression was greater in chronic DOCA-salt hypertension than in the normotensive controls.

These results demonstrate that the vascular responsiveness was increased in both acute and chronic phases of DOCA-salt hypertensive rats, while the norepinephrine overflow from the adrenergic nerve terminals was enhanced only in the chronic phase. More marked inhibition of the vasoconstrictor responses and norepinephrine overflow in the presence of a Ca-antagonist in chronic DOCA-salt hypertension might represent the higher Ca-dependency in the neurotransmission of the peripheral resistance vessels, especially in the mechanism of presynaptic norepinephrine release, than their normotensive controls, and it could partly contribute to the development and maintenance of DOCA-salt hypertension.     

Inhibition of adrenergic transmission by methionine- and leucine-enkephalins is impaired in the mesenteric vasculatures of spontaneously hypertensive rats

The present study was carried out to investigate the effects of enkephalins (methionine-enkephalin: Met-Enk, leucine-enkephalin: Leu-Enk) on the adrenergic neurotransmission in hypertension. Perfused mesenteric vasculatures were prepared in spontaneously hypertensive rats (SHR, Okamoto and Aoki strain, 7-10 weeks old) and age-matched Wistar Kyoto rats (WKY), and the effects of these peptides on vascular responsiveness as well as norepinephrine release from the sympathetic nerve endings were examined. Pressor responses to electrical nerve stimulation were inhibited in a dose-dependent manner by Met-Enk and Leu-Enk, and the inhibition was antagonized by naloxone. Norepinephrine release during electrical nerve stimulation was also suppressed by these peptides. In SHR, stimulation-evoked pressor responses and norepinephrine release were significantly enhanced compared to those in WKY, while the suppressive magnitudes of the responses by Met-Enk and Leu-Enk were smaller in SHR than in WKY. These results demonstrate that Met-Enk and Leu-Enk affected presynaptic sites of blood vessels and caused a decrease in electrically-stimulated norepinephrine release from the sympathetic nerve endings. The lower reduction in norepinephrine release and vascular responsiveness by Met-Enk and Leu-Enk in SHR suggests an insufficient regulation of the vascular adrenergic neurotransmission by the opioid peptides in this model of hypertension.     

Prostaglandin E2 inhibits noradrenaline release and purinergic pressor responses to renal nerve stimulation at 1 Hz in isolated kidneys of young spontaneously hypertensive rats

The effects of prostaglandin E2 (PGE)2 and the thromboxane A2 (TxA2) receptor agonist U-46619 on noradrenaline release and pressor responses to renal nerve stimulation (RNS) at 1 Hz were investigated in isolated kidneys of spontaneously hypertensive rats (SHR; 5-7 weeks) and age-matched Wistar-Kyoto rats (WKY). After incubation with 3H-noradrenaline, the renal nerves were stimulated. The stimulation-induced (S-I) outflow of radioactivity was taken as an index of noradrenaline release. Absolute S-I outflow of radioactivity was lower in SHR than in WKY but pressor responses to RNS were greater in SHR than in WKY. Tetrodotoxin (1 mumol/l) abolished S-I outflow of radioactivity and pressor responses to RNS in both strains. PGE2 (0.06 mumol/l) inhibited S-I outflow of radioactivity in SHR but not in WKY kidneys. PGE2 (0.6 mumol/l) inhibited S-I outflow of radioactivity in both strains. In SHR, PGE2 (0.6 mumol/l) decreased pressor responses to RNS, but increased them in WKY. In WKY, but not in SHR kidneys, pressor responses to RNS were markedly reduced by the alpha 1-adrenoceptor antagonist prazosin (0.1 mumol/l). The prazosin-resistant pressor responses to RNS were blocked by alpha, beta-methylene adenosine triphosphate (ATP; 1 mumol/l). In kidneys of SHR, pretreated with 6-OH-dopamine (50 mg/kg intravenously, 24 and 48 h before isolation of the kidneys) to destroy sympathetic nerve endings, pressor responses to RNS and S-I outflow of radioactivity were almost abolished. U-46619 (0.1 mumol/l) increased perfusion pressure in SHR and WKY kidneys and this effect was blocked by the TxA2 receptor antagonist daltroban (BM 13505; 3 mumol/l). U-46619 did not significantly modulate S-I outflow of radioactivity. The results suggest that activation of prejunctional PGE2 receptors in kidneys of SHR and WKY inhibits noradrenaline release. The prejunctional inhibitory PGE2 receptor mechanism on renal sympathetic nerves seems to operate more effectively in SHR than in WKY. There is no evidence for prejunctional TxA2 receptors in the kidneys of SHR or WKY. Pressor responses to RNS at 1 Hz in SHR kidneys seem to be due entirely to release of a purinergic co-transmitter from renal sympathetic nerves, and PGE2 possibly reduces pressor responses to RNS by inhibiting release of this purinergic co-transmitter.     

Enhanced endogenous epinephrine release from the vascular adrenergic neurons in spontaneously hypertensive rats

This study was performed to investigate the role of endogenous epinephrine in the regulation of vascular tone in hypertension. The release of endogenous epinephrine and norepinephrine from the vascular adrenergic neurons by periarterial nerve stimulation was examined in spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY). The isolated mesenteric vasculatures were prepared, and the epinephrine and norepinephrine release during electrical nerve stimulation was determined as the increase in epinephrine and norepinephrine contents in the vascular perfusate. Epinephrine and norepinephrine were measured by high performance liquid chromatography with an electrochemical detector. Vasoconstrictor responses and norepinephrine overflow during electrical nerve stimulation were significantly greater in SHR than in WKY. The amount of stimulation-evoked epinephrine overflow into the perfusate was also increased in SHR compared to that in WKY, especially at low frequency stimulation. These results suggest that epinephrine could be released from the vascular adrenergic neurons as a cotransmitter of norepinephrine and contribute to increased vascular tone in hypertension.     

Norepinephrine release and reuptake by hypothalamic synaptosomes of spontaneously hypertensive rats

We compared the overflow of endogenous norepinephrine during electrical field stimulation, the norepinephrine content, and the rate of initial neuronal uptake of [3H]norepinephrine in synaptosomes isolated from hypothalamus and brainstem of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 7 and 13 weeks of age. The synaptosomes of two rats, a SHR and a WKY rat control, were simultaneously processed and subjected to the same electrical field stimulation. The overflow of endogenous norepinephrine during electrical stimulation (2 Hz, 2 minutes) in the hypothalamic synaptosomes of 7-week-old SHR was significantly greater, whereas the overflow of 13-week-old SHR was equivalent to the age-matched WKY rat. The norepinephrine content of synaptosomes was about the same in SHR and age-matched controls. There was also significantly enhanced [3H]norepinephrine uptake in the hypothalamic synaptosomes of young SHR, but neither the hypothalamic nor the brainstem samples of 13-week-old SHR showed any significant difference in their rate of [3H]norepinephrine uptake. These data are similar to those we observed (unpublished observations) in perfused mesenteric artery system in which norepinephrine release was significantly elevated during periarterial nerve stimulation only in young SHR. Thus, these results suggest that a parallel enhancement of norepinephrine release in hypothalamus with that of peripheral nervous system may play an important role during development of hypertension in young SHR.     

Age-related decrease of calcitonin gene-related peptide-containing vasodilator innervation in the mesenteric resistance vessel of the spontaneously hypertensive rat

We previously demonstrated that the mesenteric resistance blood vessels have nonadrenergic, noncholinergic vasodilator innervation in which calcitonin gene-related peptide (CGRP) is a possible neurotransmitter. The role of CGRP-containing vasodilator nerves in hypertension was investigated in perfused mesenteric vascular beds isolated from spontaneously hypertensive rats (SHR). The adrenergic vasoconstrictor responses to perivascular nerve stimulation in both SHR (8-, 15-, and 30-week-old) and age-matched Wistar-Kyoto (WKY) rat preparations increased with aging, but the response was greater in SHR than in WKY rats at all ages. The preparation isolated from SHR and WKY rats was precontracted by continuous perfusion of Krebs' solution containing 7 x 10(-6) M methoxamine plus 5 x 10(-6) M guanethidine. In both SHR and WKY rats, perivascular nerve stimulation (1-8 Hz) produced frequency-dependent vasodilation, which was blocked by 1 x 10(-7) M tetrodotoxin, pretreatment with 5 x 10(-7) M capsaicin, and denervation by cold storage (4 degrees C for 72 hours). The vasodilation induced by perivascular nerve stimulation in SHR greatly decreased with age, whereas a slight decrease in the response with age was found in WKY rats. The neurogenic vasodilation in the young SHR preparation was similar in magnitude to the vasodilation in age-matched WKY rats, whereas the vasodilation in 15- and 30-week-old SHR was significantly smaller than that in age-matched WKY rats. In both SHR and WKY rats, perfusion of rat CGRP (1 x 10(-10) to 3 x 10(-8) M) produced marked vasodilation in a concentration-dependent manner. The CGRP-induced vasodilation in SHR increased with age, whereas an age-related decrease in vasodilation was found in WKY rats. Perivascular nerve stimulation (4 and 8 Hz) of the perfused mesenteric vascular bed evoked an increased release of CGRP-like immunoreactive substance in the perfusate, which was significantly less in 15-week-old SHR than in age-matched WKY rats. Immunohistochemical studies showed an age-related decrease in CGRP-like immunoreactive fibers in SHR but not in WKY rats. These results suggest that CGRP-containing vasodilator innervation is greatly decreased when SHR develop and maintain hypertension. It is also suggested that the decreased vasodilator mechanism by CGRP-containing nerves contributes to the development and maintenance of hypertension.     

Effects of calcium-antagonists on norepinephrine release from sympathetic nerve endings in rat mesenteric vasculature

The present study was designed to investigate whether Ca-antagonists influence norepinephrine release from the sympathetic nerve endings in resistance vessels. Isolated mesenteric vascular preparations of rats, perfused with Ringer-Locke solution, were used to determine the effects of verapamil or diltiazem on norepinephrine release from sympathetic nerve terminals. Pressor responses to exogenous norepinephrine and electrical nerve stimulation were recorded, and the norepinephrine contents of the perfusate were measured before and after the electrical nerve stimulation, using high pressure liquid chromatography with an electrochemical detector. Pressor responses to exogenous norepinephrine and electrical nerve stimulation were depressed dose-dependently by verapamil and diltiazem. The inhibition of the pressor responses for nerve stimulation was greater than exogenous norepinephrine. Both verapamil and diltiazem reduced norepinephrine release from the mesenteric arterial beds after the electrical nerve stimulation. These results indicate that Ca++-antagonists, such as verapamil and diltiazem, have inhibitory effects on norepinephrine release from the sympathetic nerve endings, in addition to their direct actions on the vascular smooth muscle. It is also suggested that the presynaptic inhibitory action of Ca++-antagonists could be due to the blockade of Ca++-channels in nerve cell terminals, which may be important in the clinical use of Ca-antagonists.     

Effects of a Protein Kinase C Inhibitor (H-7) on Norepinephrine Release from Vascular Adrenergic Neurons in Spontaneously Hypertensive Rats

This study was performed to investigate the effects of a specific protein kinase C inhibitor (H-7) on vascular adrenergic transmission in hypertension. In the isolated mesenteric vasculature of spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY), we have examined the effects of H-7 on norepinephrine (NE) release from vascular adrenergic neurons. Endogenous NE release during periarterial nerve stimulation was inhibited by H-7 in a dose-dependent manner with a concomitant reduction of pressor responses of the preparation. The inhibition of NE release was not affected by an uptake blocker of NE (desipramine). In SHR, the stimulation-evoked NE release and pressor responses were significantly greater than in age-matched WKY. The suppressive magnitude of stimulation-evoked NE release and pressor responses by H-7 were pronounced in SHR compared with WKY. These results demonstrate that endogenous NE release and pressor responses were increased in the mesenteric vasculature of SHR. Furthermore, the marked inhibition of NE release and pressor responses by H-7 in SHR may suggest the presence of enhanced protein kinase C-dependent regulation of vascular adrenergic transmission, which may contribute to the calcium-related abnormalities in this form of hypertension.     

The role of protein kinase C and calcium in the regulation of norepinephrine release from the vascular adrenergic neurons in hypertension

This study was designed to investigate the role of protein kinase C and calcium in vascular adrenergic transmission in hypertension. In perfused mesenteric vasculatures of spontaneously hypertensive rats (SHR, 7 to 10 weeks old) and age-matched Wistar-Kyoto rats (WKY), we have examined the effects of the protein kinase C inhibitor H-7 on endogenous norepinephrine release and vascular responsiveness during nerve stimulation. Endogenous norepinephrine release and pressor responses during periarterial nerve stimulation were significantly greater in SHR than in WKY. The protein kinase C inhibitor H-7 inhibited the stimulation-induced norepinephrine release and pressor responses in a dose-dependent manner. The magnitude of these suppressive responses were more pronounced in SHR than in WKY. Calcium removal from extracellular fluid also reduced the norepinephrine release more strongly in SHR than in WKY. These results demonstrate that the regulation of norepinephrine release might be more dependent on protein kinase C and calcium in the blood vessels of SHR, which could contribute, at least partially, to the pathogenesis of this form of hypertension.