Calcium antagonists and sympathetic neuroeffector function

Sympathoinhibitory and depressor effects of amlodipine in spontaneously hypertensive rats

The authors examined whether central actions contribute to the hypotensive effects of peripherally administered amlodipine, a lipophilic dihydropyridine with slow onset and long duration of action. After 5 to 6 weeks of high (8%, H-Na) or regular (0.6%, R-Na) salt intake, changes in renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP), and heart rate (HR) were recorded at rest and in response to intravenous (iv) and intracerebroventricular (icv) injection, and prolonged iv infusion of amlodipine, in conscious spontaneously hypertensive rats (SHR). Iv injection of amlodipine at 50 to 100 microg/kg decreased MAP but increased RSNA and HR in a dose-related manner. In contrast, icv injection of amlodipine at 10 to 50 microg/kg caused parallel decreases in MAP, RSNA, and HR. Iv infusion of amlodipine at 50 microg/kg per hour for 3 hours followed by 100 microg/kg per hour for 2 hours also decreased in parallel RSNA, MAP, and HR. Maximal decreases in RSNA, MAP, and HR in response to icv injection and iv infusion were significantly larger in SHR on H-Na versus R-Na. All responses lasted at least 1 hour following iv and icv injection, or after the termination of iv infusion of amlodipine. These data suggest that in SHR during prolonged iv infusion, amlodipine appears to cross the blood-brain barrier, block brain l-type Ca2+ channels, and decrease sympathetic outflow and thereby BP. Central actions may prevail during iv infusion of amlodipine at low rates, and the decrease in BP is associated with sympathoinhibition. High salt intake markedly enhances its sympathoinhibitory action, likely through central mechanisms.

Sympathoinhibitory effects of central nifedipine in spontaneously hypertensive rats on high versus regular sodium intake

-The putative central sympathoinhibitory actions of the dihydropyridine calcium antagonist nifedipine and the effect of dietary sodium on these actions were investigated in spontaneously hypertensive rats (SHR). Regular or high dietary salt was administered from 4 to 8 weeks of age. At 8 weeks, blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity were recorded in conscious rats at rest as well as in response to intravenous (50 microg/kg) and intracerebroventricular (5 and 50 microg/kg) injections of nifedipine and intracerebroventricular injections of vehicle. Resting mean arterial pressure was higher in SHR on high versus regular salt (159+/-3 versus 135+/-4 mm Hg; P<0. 05). Nifedipine administered intracerebroventricularly decreased BP as well as renal sympathetic nerve activity and HR in a dose-related manner. The responses reached their peak at 3 to 5 minutes and lasted approximately 30 minutes. Peak decreases in BP, renal sympathetic nerve activity, and HR in response to both doses of nifedipine were significantly larger in SHR on high versus regular salt. Nifedipine administered intravenously also decreased BP but, in contrast, caused (reflex) increases in renal sympathetic nerve activity and HR. On both diets, intracerebroventricular vehicle did not affect mean arterial pressure, renal sympathetic nerve activity, or HR. These data indicate that in contrast to its peripheral vasodilator effect, centrally administered nifedipine may decrease sympathetic outflow and therefore BP and HR. The enhanced sympathoinhibitory and depressor responses to nifedipine in SHR on high versus regular salt suggest that the sympathetic hyperactivity induced by high salt intake is dependent on neuronal calcium influx via L-type channels.

Cardiovascular effects of felodipine are not antagonized by dietary salt

Increased dietary intake of regular salt (sodium chloride) interferes markedly with the therapeutic effects of angiotensin converting enzyme inhibitors. To study further the interactions between dietary salt intake and antihypertensive drug treatment, we examined the effects of felodipine, a dihydropyridine derivative Ca2+ channel antagonist with natriuretic properties, on blood pressure and the development of left ventricular hypertrophy in the stroke-prone spontaneously hypertensive rats during different levels of sodium chloride in the diet. We also compared the influence of regular salt on the cardiovascular effects of felodipine with that of a novel K(+)-, Mg(2+)- and l-lysine-enriched and Na(+)-reduced salt alternative, which in previous studies markedly improved the therapeutic effects of enalapril and ramipril. During the 28-day experiment regular salt produced a marked rise in blood pressure and induced left ventricular hypertrophy, while the salt alternative neither induced any rise of blood pressure nor caused cardiac hypertrophy. Felodipine had an enhanced antihypertensive effect during the increased intake of sodium chloride, and lowered the blood pressure to the same normotensive level as it did during the control and the salt alternative diets. Felodipine also completely blocked the development of the sodium chloride-induced cardiac hypertrophy. The heart rate of the felodipine-treated animals was significantly increased during the first two study weeks but thereafter it did not differ from that of the controls. Hence, unlike regular salt, the novel Na(+)-reduced, K(+)-, Mg(2+)-, and l-lysine-enriched salt alternative did not raise blood pressure and produced little if any left ventricular hypertrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium dependence of contractile activation of isolated sheep urethra. I: Responses to electrical stimulation

In isolated sheep urethral smooth muscle at resting tension, responses elicited by electrical stimulation of nerves were either contraction (40%), or contraction preceded by relaxation (60%). All responses were suppressed by tetrodotoxin (TTX), but at stimulation frequencies exceeding 16 Hz, there was a small TTX resistant contraction, which at the highest frequency used (50 Hz) amounted to about 15% of the total response. alpha-Adrenoceptor blockade suppressed the contractions, as did chemical sympathectomy with 6-hydroxydopamine. In calcium-free medium all electrically induced contractions were abolished. Nifedipine, verapamil, and lanthanum had concentration-dependent depressant actions, whereas Bay K 8644 (10(-6) M) significantly increased the responses. In preparations prelabelled with 3H-noradrenaline, electrical stimulation caused a release of 3H that could be blocked by TTX, and effectively reduced by calcium-free medium and lanthanum. However, the release was unaffected by nifedipine, and moderately reduced by high concentrations of verapamil (10(-5) M). It is suggested that contractile responses to electrical stimulation in isolated sheep urethral smooth muscle are mediated by the sympathetic nervous system, mainly through release of noradrenaline stimulating postjunctional alpha 1-adrenoceptors. The release of noradrenaline, as well as the action of released noradrenaline on postjunctional membranes are calcium-dependent. However, the calcium entry pathways used are partly different from those inhibited by organic calcium antagonists.

Protective effects of felodipine and verapamil against imipramine-induced lethal cardiac conduction disturbances in the anaesthetized rat

The effect of verapamil and felodipine on imipramine-induced cardiac toxicity was assessed in anaesthetized rats. Rats received either infusion of saline (n = 13), or non-hypotensive doses of felodipine (n = 36) or verapamil (n = 36) over 40 minutes. In saline-pretreated rats IV bolus injection of imipramine (10 mg/kg) resulted in severe hypotension, bradycardia, electromechanical dissociation, and death within 3 minutes. Rats pretreated with nonhypotensive doses of felodipine (1, 3, 6, and 10 micrograms/kg/min) or verapamil (10, 30, 100, and 300 micrograms/kg/min) survived throughout the experiment, despite an initial fall in blood pressure within the first 5 minutes after imipramine administration. Blood pressure returned to baseline levels within 15 minutes. Intermittent ECG monitoring showed significant prolongation of QRS duration after imipramine in both saline (by 138%) and verapamil-pretreated rats (by 63%), whereas in the rats pretreated with felodipine no prolongation was observed (+3%). We conclude that, in our experimental model, felodipine, more than verapamil, protects against the cardiac effects of imipramine intoxication.

Effects of nimodipine, Bay K 8644 and pinacidil on alpha 1- and alpha 2-adrenoceptor-mediated vasoconstriction in human hand veins

The effects of nimodipine, Bay K 8644 and pinacidil, three drugs interfering with transmembrane Ca2+ fluxes in different ways, were investigated in isolated human hand veins. Their ability to influence the concentration-response relationship for noradrenaline (NA) was assessed in the absence and presence of prazosin or rauwolscine. The contractile response to NA was almost abolished in Ca2+ -free medium. Nimodipine and pinacidil depressed the NA concentration-response curve both in the absence and presence of alpha-adrenoceptor blockers. The NA response was only partially inhibited by nimodipine, indicating that NA may activate nimodipine-insensitive influx pathways, presumably receptor-operated calcium channels. Pinacidil inhibited the contractile response to 124 mM K+ and reduced the NA-induced contraction in the presence of nimodipine, suggesting that pinacidil has actions other than the opening of potassium channels and subsequent membrane hyperpolarization. Bay K 8644 increased the NA potency fourfold in the presence of rauwolscine, whereas it had no effect on the NA response in the presence of prazosin and in the absence of alpha-adrenoceptor blockade. Such an action of Bay K 8644 can be reconciled with alpha 1-adrenoceptor activation causing membrane depolarization and opening of potential-operated calcium channels. It may be concluded that both alpha 1- and alpha 2-adrenoceptor-mediated contractions in human hand veins are highly dependent on Ca2+ influx, although the mechanisms utilized to bring about this influx partly differ between the two receptor subtypes.

Studies on the anti-vasoconstrictor activity of BRL 34915 in spontaneously hypertensive rats; a comparison with nifedipine

1. The blood pressure lowering and anti-vasoconstrictor effects of BRL 34915 and nifedipine were compared in female spontaneously hypertensive rats (SHR). 2. In conscious SHR, intravenous injection of BRL 34915 (0.1, 0.3 mg kg-1) produced rapid, dose-related falls in mean arterial pressure of greater than 3 h duration. Nifedipine, at the same intravenous dose levels, also evoked rapid anti-hypertensive effects, though these responses were of lesser magnitude and duration than those observed for BRL 34915. 3. In anaesthetized, ganglion-blocked SHR, BRL 34915 (0.1, 0.3 mg kg-1 i.v.) dose-dependently antagonized the pressor responses to incremental intravenous infusions of noradrenaline (3.8-28.5 ng min-1) or phenylephrine (120-907 ng min-1) but did not inhibit pressor responses to incremental infusions of methoxamine (0.47-3.63 micrograms min-1), angiotensin II (7.0-52.9 ng min-1) or vasopressin (0.27-2.0 mu min-1). 4. In anaesthetized, ganglion-blocked SHR, nifedipine (0.1, 0.3 mgkg-1 i.v.) antagonized the pressor responses to each of the infused vasoconstrictor agents, being most effective against responses to noradrenaline or angiotensin II. 5. In pithed SHR, both BRL 34915 and nifedipine (each at 0.3 mg kg-1 i.v.) reduced the basal blood pressure level and produced marked inhibition of frequency-dependent pressor responses evoked by electrical stimulation of the spinal cord sympathetic outflow (0.25-4.0 Hz). Restoration of the basal diastolic blood pressure to within the control range, using a continuous intravenous infusion of vasopressin (0.98 mu min-1), prevented the inhibitory effect of BRL 34915. In the case of nifedipine, however, even raising the basal blood pressure to a level exceeding that recorded in control rats (with vasopressin, 2.0 mu min-1), did not reverse the inhibitory effect of the drug on frequency-dependent pressor responses. 6. It is concluded that the anti-hypertensive properties of BRL 34915 in SHR are probably unrelated to an anti-vasoconstrictor action. In contrast, it is suggested that the broadly-based anti-vasoconstrictor properties of nifedipine may contribute substantially to the anti-hypertensive properties of this drug.

Calcium channel blockade and contractile responses in the isolated human vas deferens

The effects of removal of extracellular calcium and of the calcium channel blockers nifedipine, verapamil and diltiazem were studied on contractions induced by electrical field stimulation and high K+-solution in isolated preparations of the human vas deferens. Electrically induced contractions were blocked by tetrodotoxin and alpha-adrenoceptor blockade. They were abolished in calcium-deficient medium, and suppressed by the calcium channel blockers in the order of potency nifedipine greater than verapamil greater than diltiazem. The maximum blocking effect of nifedipine was approximately 40%. All the blockers practically abolished K+-induced contractions. It is concluded that even if the contractile response of the human vas deferens to electrical stimulation is dependent on extracellular calcium, calcium channel blockers seem to have only a limited effect on this contraction and their capability of impairing the function of the vas deferens in patients is questioned.