Effects of neuropeptide Y on norepinephrine release in hypothalamic slices of spontaneously hypertensive rats

We describe the effects of neuropeptide Y (NPY) on [3H]norepinephrine (NE) release from hypothalamic slices of spontaneously hypertensive rats (SHR). The electrical stimulation (1 Hz)-evoked [3H]NE release was significantly greater in hypothalamic slices of SHR than in those of Wistar Kyoto rats (WKY). NPY inhibited the stimulation-evoked [3H]NE release in a dose-dependent manner. The inhibitory effect of NPY was significantly attenuated in SHR compared with WKY. The results may indicate that the less inhibitory effect of NPY on NE release induces increased sympathetic nerve activity in the hypothalamus of SHR.     

Vessel reactivity and prejunctional modulatory changes in the portal vein of mature spontaneously hypertensive rats

The prejunctional effects of 2-chloroadenosine on the contractile responses to perivascular nerve stimulation were studied in conjunction with vessel reactivity in portal veins from mature (45-60 weeks) spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls. It was found that the contractility of the portal veins to exogenous noradrenaline was enhanced in SHR, while the sensitivity to noradrenaline, as seen from the EC50 values, was not altered. Responses to perivascular nerve stimulation (supramaximal voltage, 0.7 ms pulse duration for 10 s) were also enhanced in spontaneously hypertensive rats compared to WKY controls at all frequencies of stimulation tested (2-64 Hz), although the sensitivity to perivascular nerve stimulation was not changed since a response of 50% of the maximal obtainable was achieved at 8 Hz in both SHR and WKY. However, no evidence for change in the level of prejunctional modulation of nerve stimulation responses or postjunctional modulation of noradrenaline responses by 2-chloroadenosine (0.1-100 microM) was found. In conclusion, in mature SHR, responses to perivascular nerve stimulation and exogenous noradrenaline were found to be enhanced compared to responses in WKY, although there was no difference in the modulatory action of 2-chloroadenosine between SHR and WKY.     

Effects of endothelin on the portal vein from spontaneously hypertensive and Wistar Kyoto rats.

The effects of endothelin, a novel potent vasoconstrictor peptide, on isolated portal veins were examined in spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). Endothelin contarcted the portal vein from SHR and WKY, in a concentration-dependent manner. However, both twitch contraction and tonic contraction of portal veins in response to endothelin were significantly enhanced in SHR. In contrast to the effects of endothelin, twitch contractile responses to Bay K 8644 were not significantly different between vessels from SHR and WKY. These results indicate that endothelin is a potent vasoconstrictor peptide in the portal vein, and that the increased sensitivity in SHR may be due to an increase in the activity of voltage-dependent Ca2+ channels which is modulated by endothelin, but not to an increase in the activity of voltage-dependent Ca2+ channels which can be stimulated by Bay K 8644.     

Effects of azimilide on cardiovascular tissues from normo- and hypertensive rats

We tested whether azimilide has potential for use in the treatment of heart failure. Azimilide, > or =3 x 10(-5) M, had no effect on the quiescent Wistar-Kyoto (WKY) rat aorta, or mesenteric and intralobar pulmonary arteries. Azimilide > or =3 x 10(-5) M relaxed the KCl-contracted aorta and portal vein. Azimilide, 10(-7)-10(-5) M, prolonged the WKY left ventricular action potential and augmented the force of contraction of left ventricle strips from 12- and 22-month-old WKY rats. Spontaneously hypertensive rats (SHRs), at ages 12 and 22 months, are models of cardiac hypertrophy and failure, respectively. The augmentation of force with azimilide was similar on 12- and 22-month-old WKY rats and 12-month-old SHRs but reduced on the 22-month-old SHR left ventricle. Azimilide, 3 x 10(-6) and 10(-5) M, augmented the force responses of the 22-month-old SHR left ventricle by 40 and 50%, respectively. As azimilide is a vasodilator and positive inotrope in the rat, and the positive inotropic effect is present in heart failure, azimilide should undergo further testing as a positive inotrope for the treatment of heart failure.     

Effects of dofetilide on cardiovascular tissues from normo- and hypertensive rats

The aim was to test whether dofetilide has some potential for use in the treatment of heart failure. Dofetilide at < or = 3 x 10(-5) M had no effect on the quiescent Wistar Kyoto (WKY) rat aorta, mesenteric and intralobar arteries, or the spontaneous contractions of the WKY rat portal vein. Dofetilide at 10(-6) to 3 x 10(-5) M relaxed the KCl-contracted aorta. Dofetilide at 10(-9)-10(-7) M augmented the force of contraction of leftventricle strips from 12- and 18-month-old WKY rats at 2 Hz. Spontaneously hypertensive rats (SHRs) at 12 and 17-21 months of age are models of cardiac hypertrophy and failure, respectively. The augmentation of force at 2 Hz with dofetilide was similar on 12- and 18-month-old WKY rats and 12-month-old SHRs but reduced on the 18-month-old SHR left ventricle. At a higher more physiological frequency, 4 Hz, the threshold concentration of dofetilide required to augment the force responses of 21-month-old SHR left ventricles was markedly increased and the maximum augmenting effect was decreased. Dofetilide at 10(-7)-10(-5) M reduced the rate of the 17-month-old WKY rat right atrium, and had a similar effect on age-matched SHR right atrium. In summary, dofetilide is a positive inotrope and negative chronotrope in the rat. However, as the positive inotropic effect is not observed with clinically relevant concentrations at a physiological rate in heart failure, dofetilide is unlikely to be useful as a positive inotrope in the treatment of heart failure.     

Effects of clofilium on cardiovascular tissues from normo- and hypertensive rats

1. The overall aim was to test whether clofilium has some potential as a positive inotrope for heart failure. We used Wistar Kyoto normotensive rats (WKY) and spontaneously hypertensive rats (SHRs) and studied the effects of clofilium on isolated blood vessels, left ventricular action potentials and left ventricular contractility. 2. Clofilium at < or = 10(-6) M had no effect on WKY portal vein contractions and at < or = 3 x 10(-4) M had no effect on WKY or SHR quiescent mesenteric and intralobar pulmonary arteries. 3. Clofilium at 10(7) - 10(-5) M prolonged the WKY left ventricular action potentials and with 10(-6) and 10(-5)M this included after-depolarizations. 4. Clofilium at < or = 3 x 10(-5) M augmented the peak force, prolonged the contractions and did not cause arrhythmias in the absence and presence of isoprenaline on left ventricle strips from 12-month-old WKY. 5. The 12-month-old SHR has hypertrophy of the left ventricle with reduced peak force and prolongation of relaxation. The effects of clofilium on 12-month-old SHR left ventricle contractility were similar to those in the age-matched WKY. 6 In summary, clofilium has positive inotropic effects on the rat left ventricle that are maintained in hypertrophy. Clofilium does not have effects on blood vessels that would be detrimental in heart failure. Clofilium prolongs the rat left ventricle action potential and causes after-depolarizations. The pro-arrhythmic potential of clofilium, however, makes it unlikely that it could be used as a positive inotrope in the treatment of heart failure.     

Responsiveness, affinity constants and beta 2-adrenoceptor reserves for isoprenaline on portal veins from normo- and pre- and hypertensive rats

1. This study used contractility methods with the portal veins of 5- and 14-week-old Wistar-Kyoto normotensive rats (WKY) and spontaneously hypertensive rats (SHRs). The SHRs are prehypertensive at 5 weeks. 2. The first part of our study was to determine whether the responsiveness to isoprenaline and forskolin was altered in the maturation of portal veins from normo- and prehypertensive rats. The responses to forskolin were similar on the portal veins of 5- and 14-week-old WKY and SHRs. 3. The sensitivity and maximum responses to isoprenaline were similar on portal veins of 5- and 14-week-old WKY. The sensitivity and maximum responses to isoprenaline were lower on the portal veins of 5-week-old SHRs (pD2 = 8.25, maximum = 85%) than age-matched WKY (pD2 = 8.79, maximum = 96%); these differences are not caused by hypertension. At 14 weeks, the sensitivity was similar (WKY pD2 = 8.74, SHR pD2 = 8.65) but the maximum responses to isoprenaline were lower on the portal veins SHRs (77%) than WKY (97%). Thus, the sensitivity to isoprenaline increases with the development of hypertension in the SHR portal vein. 4. The second part of the study was to determine whether the affinity for isoprenaline at beta2-adrenoceptors and the fractional beta2-adrenoceptor occupancy-response relationships on the portal vein were altered in maturation from normo- and pre-hypertensive rats. The effects of bromoacetylalprenololmenthane (BAAM), an irreversible beta-adrenoceptor blocker, on the isoprenaline responses of 5- and 14-week-old WKY and SHRs were studied. Maturation of the WKY portal vein between 5 and 14 weeks was associated with a loss of affinity for isoprenaline (from pKA of 7.13 to 7.87), and increase in beta2-adrenoceptor reserve (from 72 to 92% at the 95% response). There were similar affinity and reserve findings in the maturation of the SHR portal vein. Thus, there are major changes in beta2-adrenoceptor structure and reserve in maturation on the portal vein that are irrespective of the development of hypertension.     

The effect of diltiazem on noradrenaline release.

The effects of diltiazem in rat tail arteries and guinea-pig vasa deferentia have been investigated. Superfusion of the rat tail artery with diltiazem (10(-6) - 10(-4) M) resulted in a dose-related increase in 3H-overflow (P less than 0.001) both in Wistar Kyoto (WKY) and in spontaneously hypertensive (SHR) rats. Release of 3H by transmural stimulation (1 Hz, 2 ms, 10 V) was also much greater in vessels perfused with diltiazem; this effect was dose-dependent. Diltiazem did not significantly alter the proportion of noradrenaline and its metabolites in 3H-overflow, as analysed by column chromatography. In the vasa deferentia of guinea-pigs, diltiazem (10(-9) - 10(-5) M) increased spontaneous 3H-release. The results indicate that diltiazem acts on sympathetic nerves and causes the release of noradrenaline.     

CONTRACTILE RESPONSES TO α1-ADRENOCEPTOR STIMULATION DURING MATURATION IN THE AORTA OF THE NORMOTENSIVE AND SPONTANEOUSLY HYPERTENSIVE RAT: RELATION TO STRUCTURE

1. The significantly greater rise in blood pressure during the first 20 weeks of life in spontaneously hypertensive rats (SHR) when compared with normotensive Wistar-Kyoto rats (WKY) may be related to increased vasoconstrictor responses caused by enhanced transmitter release or post-junctional receptor changes. 2. The reactivity of rat isolated aorta to post-junctional alpha 1-adrenoceptor stimulation by methoxamine and to transmural sympathetic nerve stimulation was studied in ring segments suspended at equivalent transmural pressures in organ baths. 3. Wall stress in the SHR aorta was significantly higher at 4 weeks, but lower at 9 and 20 weeks when compared with the WKY aorta, a possible adaptation to the higher pressures seen in the SHR at the latter ages. 4. The sensitivity (location of EC50) to methoxamine was similar at all ages in both strains, but the SHR aortae at 9 and 20 weeks generated higher maximal contractile force to this agent compared with the WKY aorta. 5. The increase in force to methoxamine parallelled the medial hypertrophy of the SHR aorta, determined from computerized morphometric analysis. 6. There was an enhanced response to transmural field stimulation in the SHR aortae at 9 weeks, that was not accounted for by medial hypertrophy or altered neuronal uptake of noradrenaline. 7. These studies suggest that enhanced maximal contractile force in the SHR aorta to alpha 1-adrenoceptor stimulation is accounted for by medial hypertrophy. However, there is an additional enhanced reactivity at 9 weeks in response to nerve stimulation in the SHR aorta that may be related to increased innervation density.     

Vascular inhibitory actions of 17β‐oestradiol in rat portal vein

1 We have examined the effects of 17β‐oestradiol on spontaneous spikes and contractions to noradrenaline in rat portal vein. 2 The 17β‐oestradiol (10 μM ) significantly reduced the maximum height of spontaneous spike contractions and significantly reduced the maximum contraction to noradrenaline (10 μM ) in portal vein from both male and female rats. 3 The protein synthesis inhibitor cycloheximide (10 μM ) did not significantly affect the inhibitory actions of 17β‐oestradiol (10 μM ) against spontaneous spike contractions and contractions to noradrenaline. 4 In summary, 17β‐oestradiol diminishes spontaneous spike contractions and the contractile response to noradrenaline in rat portal vein, by a non‐genomic action.     

α₂-Adrenoceptor-mediated inhibition of catecholamine release from the adrenal medulla of spontaneously hypertensive rats is preserved in the early stages of hypertension

In this study, we evaluated the effect of α(2) -adrenoceptor activation on catecholamine release from the adrenal medulla of pre-hypertensive (6-week-old) and hypertensive (16-week-old) spontaneously hypertensive rats (SHR) and of age-matched normotensive control Wistar Kyoto (WKY) rats. Catecholamine overflow from isolated adrenal medullae was evoked by the nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) in the absence and presence of the α(2) -adrenoceptor agonist medetomidine (MED). The spontaneous outflow of adrenaline was similar between age-matched SHR and WKY rats. However, the spontaneous outflow of noradrenaline was significantly lower in SHR compared with age-matched WKY rats. DMPP (0.1-3 mM) increased the outflow of noradrenaline and adrenaline in a concentration-dependent manner. The E(max) values for adrenaline overflow were similar between strains, but the E(max) values for noradrenaline overflow were significantly lower in SHR. The EC(50) values for noradrenaline and adrenaline overflow were significantly higher in SHR compared with age-matched WKY rats. MED (0.1-300 nM) reduced the DMPP-evoked overflow (DMPP 500 μM) of noradrenaline and adrenaline in a concentration-dependent manner and was capable of totally inhibiting this effect. The inhibitory action of MED was similar between age-matched SHR and WKY rats. In the adrenals, the α(2A)- and α(2B)-adrenoceptor subtypes had the highest mRNA expression levels; the α(2C)-adrenoceptor subtype had the lowest mRNA expression levels. The mRNA levels for the three subtypes were similar between strains. In conclusion, in SHR during the development of hypertension, adrenal α(2) -adrenoceptor inhibitory function is conserved, accompanied by reduced noradrenaline release and unchanged adrenaline release.     

Adrenergic and purinergic components in bisected vas deferens from spontaneously hypertensive rats

1. Purinergic and adrenergic components of the contractile response to electrical field stimulation (EFS) have been investigated in epididymal and prostatic portions of Wystar Kyoto (WKY) and spontaneously hypertensive rat (SHR) vas deferens. 2. In both halves of SHR and WKY vas deferens, EFS (40 V, 0.5 ms for 30 s, 0.5-32 Hz) evoked frequency-related contractions. The neurogenic responses were biphasic, consisting of a rapid non-adrenergic response, dominant in the prostatic portion, followed by a slow tonic adrenergic component, dominant in the epididymal half. 3. Phasic and tonic components of the frequency-response curves evoked by EFS were significantly higher in the epididymal but not in the prostatic portion of vas deferens from SHR compared to WKY rats. 4. The alpha1-adrenoceptor antagonist prazosin (0.1 microM) was more effective against both components of the contractile response in the epididymal end of SHR than in WKY rats. 5. Inhibition by alpha, beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP 3 and 30 microM) was higher in both components of the contractile responses in WKY preparations than in SHR. 6. Combined alpha1-adrenoceptor and P2x-purinoceptor antagonism virtually abolished the EFS-evoked contractile response in both strains. The degree of inhibition by prazosin (0.1 microM) after P2x-purinoceptor blockade was higher in SHR than in WKY rats. 7. These results demonstrate a modification in the purinergic and noradrenergic contribution to neurogenic responses in SHR and WKY animals besides a co-participation of ATP and noradrenaline in both contractile components of the response to EFS.     

Effects of tetraethylammonium, 4-aminopyridine and bretylium on cardiovascular tissues from normo- and hypertensive rats.

Our objective was to test whether potassium-channel blockade is a potential positive inotropic mechanism for heart failure. Thus we studied the effects of tetraethylammonium, 4-aminopyridine and bretylium on left ventricular action potentials, left ventricular contractility in the absence and presence of hypertrophy, and on isolated blood vessels from Wistar Kyoto normotensive rats (WKY) and spontaneously hypertensive rats (SHRs). Tetraethylammonium at 10(-3)-10(-2) M, 4-aminopyridine at 10(-4)-10(-3) M and bretylium at 10(-6)-10(-4) M prolonged the action potentials of the WKY left ventricular strip. Similar concentrations of tetraethylammonium, 4-aminopyridine and bretylium augmented the peak force, prolonged the contractions, and did not cause arrhythmias in the absence or presence of isoprenaline on left ventricular strips from 12-month-old WKY. The 12-month-old SHR has hypertrophy of the left ventricle with reduced contractility and prolongation of relaxation. The effects of tetraethylammonium and bretylium were similar on WKY and SHR, whereas the effects of 4-aminopyridine were reduced on SHR left ventricular contractility, which suggests that the function of the transient outward-blocking potassium channel may be impaired in hypertrophy. Bretylium at < or = 10(-4) M had no effect on the portal vein, intralobar or mesenteric arteries. Tetraethylammonium and 4-aminopyridine at > or = 10(-5) M increased the duration or amplitude, or both, of the portal vein contractions. Tetraethylammonium at > or = 10(-2) M and 4-aminopyridine at > or = 3 x 10(-4) M contracted the mesenteric artery, and 4-aminopyridine also contracted the intralobar pulmonary artery. In summary, we have demonstrated that the action potential prolonging effects of potassium-channel blockade is associated with a positive inotropic effect on the rat left ventricle. The non-specific blockers, tetraethylammonium and 4-aminopyridine, do not have potential as positive inotropes in heart failure because of their widespread effects, including vasoconstriction. The potential of bretylium and some of the newer selective potassium-channel blockers as positive inotropes requires further evaluation.     

Effects of Bay K 8644 and nifedipine on femoral arteries of spontaneously hypertensive rats.

Vasoconstrictor effects of Bay K 8644 (an agonist known to increase Ca2+ influx through the voltage-dependent Ca2+ channels) on femoral arteries of 6 week old spontaneously hypertensive rats (SHR) were investigated, and data compared with findings in age-matched normotensive Wistar-Kyoto rats (WKY). The addition of Bay K 8644 (1 X 10(-10)-3 X 10(-7) M) elicited a dose-dependent contraction in SHR femoral artery in the absence of any contractile agent. Maximum contraction induced by this agonist was the same as the maximum induced by either K+-depolarization or alpha-adrenoceptor stimulation. Bay K 8644 was less effective in eliciting a contraction in the WKY femoral artery. Increased sensitivity to K+ was also observed in the SHR femoral artery. In contrast, contractions in response to alpha-adrenoceptor stimulation were the same in the SHR as those in the WKY. The addition of nifedipine, a Ca2+ channel antagonist, to an unstimulated preparation produced a dose-dependent relaxation in femoral arteries from SHR, but not from WKY. When the arteries were contracted with 60 mM K+, nifedipine produced similar relaxations in the SHR as those in the WKY, suggesting that the Ca2+ channels in the SHR femoral arteries are more activated than those in the WKY femoral arteries. Contractile responses to SHR femoral arteries to Bay K 8644 were antagonized competitively by nifedipine. Contractile responses to Ca2+ determined in K+-depolarized strips were also antagonized competitively by nifedipine. However, Schild plot analysis demonstrated a different pA2 value for nifedipine, suggesting that there may be a difference in the state of voltage-dependent Ca2+ channels in SHR femoral artery between the stimulation with Bay K 8644 and K+-depolarization.     

Comparison of the purinergic contribution to sympathetic vascular responses in SHR and WKY rats in vitro and in vivo

Isolated tail arteries from spontaneously hypertensive rats (SHR) were more responsive than those from Wistar-Kyoto (WKY) control rats to exogenously applied noradrenaline (NA), ATP, alpha,beta-methylene ATP (mATP), KCl and sympathetic nerve stimulation. The sympathetic contractile responses of the SHR and WKY were both reduced to 10-20% of control by alpha 1-adrenoceptor antagonism. The pressor responses to sympathetic nerve stimulation were significantly greater in the SHR than the WKY rats at all stimulation frequencies examined (1-10 Hz). There was no significant difference between SHR and WKY rats in the magnitude of pressor responses produced by i.v. administration of NA or mATP. The pressor responses to sympathetic nerve stimulation in the pithed SHR were no more resistant to alpha-adrenoceptor antagonism than those of the WKY. The results suggest that the contribution by ATP to sympathetic vasoconstriction is no greater in SHR than WKY.     

Alpha 1-adrenoceptor reserve and effects of a Ca2+ entry blocker (Ro 18-3981) on aorta of spontaneously hypertensive rats

The relationship between alpha 1-adrenoceptor reserve and the sensitivity of vasoconstrictor responses to Ca2+ entry blockade was investigated in isolated aortas from age-matched (13-15 weeks) spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Noradrenaline (NA) elicited contractile responses with a greater potency (log EC50) in aorta from WKY (-8.7) than in those from SHR (-8.05). The dihydropyridine Ca2+ entry blocker, Ro 18-3981 (10(-6) M), suppressed the maximal NA responses more in aorta of SHR (-54%) than WKY (-14%). The dissociation constant (KA) of NA was similar in aortas of both strains. However, the difference between KA and EC50 values was greater in aorta of WKY (7.2 X) than in those from SHR (1.4 X). Pretreatment of WKY aorta with the irreversible alpha-blocker phenoxybenzamine (10(-9) M) enhanced the inhibitory effect of Ro 18-3981 (10(-6) M) against NA-induced contractions (-14 to -47%). Thus, a smaller alpha 1-adrenoceptor reserve could explain the greater sensitivity of NA-induced contractions in SHR aorta to Ca2+ entry blockade.     

Effects of clofilium on cardiovascular tissues from normo‐ and hypertensive rats

1 The overall aim was to test whether clofilium has some potential as a positive inotrope for heart failure. We used Wistar Kyoto normotensive rats (WKY) and spontaneously hypertensive rats (SHRs) and studied the effects of clofilium on isolated blood vessels, left ventricular action potentials and left ventricular contractility.
2 Clofilium at ≤10^?6 m had no effect on WKY portal vein contractions and at ≤3×10?4 m had no effect on WKY or SHR quiescent mesenteric and intralobar pulmonary arteries.
3 Clofilium at 10^?7–10^?5 m prolonged the WKY left ventricular action potentials and with 10^?6 and 10^?5 m this included after‐depolarizations.
4 Clofilium at ≤3×10^?5 m augmented the peak force, prolonged the contractions and did not cause arrhythmias in the absence and presence of isoprenaline on left ventricle strips from 12‐month‐old WKY.
5 The 12‐month‐old SHR has hypertrophy of the left ventricle with reduced peak force and prolongation of relaxation. The effects of clofilium on 12‐month‐old SHR left ventricle contractility were similar to those in the age‐matched WKY. 6 In summary, clofilium has positive inotropic effects on the rat left ventricle that are maintained in hypertrophy. Clofilium does not have effects on blood vessels that would be detrimental in heart failure. Clofilium prolongs the rat left ventricle action potential and causes after‐depolarizations. The pro‐arrhythmic potential of clofilium, however, makes it unlikely that it could be used as a positive inotrope in the treatment of heart failure.     

Effect of sodium depletion on the release of [3H]norepinephrine from central and peripheral tissue of Wistar-Kyoto and spontaneously hypertensive rats

To study the relationship between sodium intake, the sympathetic nervous system, and hypertension, we studied the effects of a 7-9 day dietary restriction of sodium in three different ages of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Field-stimulated [3H]norepinephrine ( [3H]NE) release was measured in portal vein, anterior hypothalamus, and the A2 region of the nucleus tractus solitarius (NTS) of 5- to 6-, 10- to 11-, and 28- to 30- week-old SHR and age-matched WKY. A low-sodium diet (0.05% Na+, control 0.5% Na+) significantly lowered stimulated [3H]NE release from portal vein and anterior hypothalamus in SHR and WKY at all three ages. However, release from the A2 region was not altered by sodium restriction. The results of the present study suggest that lowered dietary sodium can selectively alter norepinephrine release in both the peripheral and central sympathetic nervous system of SHR and WKY. The results also suggest that the SHR at 5-6 weeks are more sensitive to altered dietary sodium than are age-matched WKY.