Endogenous angiotensin II and bradykinin delay and attenuate the hypotension after N-type calcium channel blockade in conscious rabbits

Hypotension and ischaemic stroke associated with aliskiren in the ALTITUDE trial: sensitisation of the Bezold-Jarisch reflex?

Hypotension and syncopal attacks have been reported in association with drugs which block the renin-angiotensin-aldosterone system (RAAS). It has been proposed that the underlying mechanism is due to sensitisation of the Bezold-Jarisch reflex leading to withdrawal of sympathetic tone, profound and prolonged bradycardia, and hypotension. Sensitisation of this reflex occurs in the presence of blockade of the RAAS. In the ALTITUDE trial the use of the direct renin inhibitor, aliskiren, was associated with hypotensive episodes and an excess of ischaemic stroke. It is hypothesised that this is best explained by activation of the Bezold-Jarisch reflex, which may be particularly important in circumstances where there is dual blockade of the RAAS.

Bradykinin B2 receptor-dependent enhancement of enalapril-evoked hypotension in ethanol-fed female rats

Our previous studies showed that chronic ethanol feeding attenuates centrally (clonidine) evoked and potentiates peripherally (hydralazine) evoked hypotension in female rats. In this study, we investigated whether chronic ethanol (8 weeks, 5% wt/vol) alters hemodynamic responses elicited by angiotensin-converting enzyme (ACE) inhibition (enalapril) in telemetered female rats. Given the intimate interaction between ACE and bradykinin, studies were extended to investigate the role of bradykinin receptor (B2R) in ethanol-enalapril interaction. Compared with pair-fed controls, ethanol-fed female rats exhibited (1) higher renal expressions of ACE and B2R proteins and angiotensin II levels and (2) lower blood pressure. Pharmacological inhibition of ACE and B2R supports functional role for the higher levels of these 2 proteins in ethanol-fed rats because enalapril (10 mg/kg, intraperitoneally) caused significantly greater hypotensive response in ethanol-fed rats than in control rats. Further, blockade of B2R with bradyzide (2 mg/kg, intraperitoneally) abrogated the enhanced hypotensive effect of enalapril in ethanol-fed rats but had no effect on enalapril-evoked hypotension in control rats. Finally, enalapril enhancement of spontaneous baroreflex sensitivity (BRS) in control was absent in ethanol-fed rats. These findings demonstrate that chronic ethanol produces B2R-dependent enhancement of the hypotensive response elicited by enalapril and abrogates enalapril-evoked enhancement of spontaneous baroreflex response in female rats.

Central endogenous histamine modulates sympathetic outflow through H3 receptors in the conscious rabbit

1. This study examined the role of histamine H(3) receptors in vagal and sympathetic autonomic reflexes in the conscious rabbit, and in rabbit and guinea-pig isolated right atria. 2. The baroreceptor-heart rate reflex (baroreflex), Bezold-Jarisch-like and nasopharyngeal reflexes were assessed after these treatments (i.v.; with H(1) and H(2) receptor block): (i) vehicle (saline; n=11); (ii) H(3) receptor agonist, (R)-alpha-methylhistamine (R-alpha-MH) 100 micro g kg(-1)+100 micro g kg(-1) h(-1) (n=9); (iii) H(3) receptor antagonist, thioperamide 1 mg kg(-1)+1 mg kg(-1) h(-1) (n=11); (iv) R-alpha-MH and thioperamide (n=6); and (v) H(2) and H(3) antagonist, burimamide 6.3 mg kg(-1)+6.3 mg kg(-1) h(-1) (n=4). 3. R-alpha-MH caused a thioperamide-sensitive fall in mean arterial pressure (MAP) of 8+/-1 mmHg and tachycardia of 18+/-2 bpm (P<0.0005). Burimamide was without effect, however thioperamide elicited an increase in MAP of 4+/-1 mmHg (P<0.01), but no change in heart rate (HR). 4. R-alpha-MH caused a 44% decrease in the average gain of the baroreflex (P=0.0001); this effect was antagonised by thioperamide. Thioperamide caused a parallel rightward shift in the barocurve with an increase in MAP of 5 mmHg (P<0.05). Burimamide had no effect on the baroreflex. The vagally mediated bradycardia elicited by the Bezold-Jarisch and nasopharyngeal reflexes was unaffected by H(3) receptor ligand administration. 5. R-alpha-MH (<or=10 micro M) caused a thioperamide-sensitive depression of both sympathetic and vagal responses in guinea-pig atria, but had no effect in rabbit atria. 6. As H(3) receptor activation caused a significant decrease in baroreflex gain without affecting HR range, the former is unlikely to be simply due to peripheral sympatholysis (supported by the lack of effect in isolated atria). Central H(3) receptors may have a tonic role in the baroreflex as thioperamide caused a rightward resetting of the barocurve. In contrast, the peripherally acting H(3) antagonist burimamide was without effect. These findings suggest a role for central histamine H(3) receptors in cardiovascular homeostasis in the rabbit.

Acute effects of L- and T-type calcium channel antagonists on cardiovascular reflexes in conscious rabbits

1. The effects of the relatively selective T-type voltage- operated calcium channel (VOCC) antagonist mibefradil were compared with verapamil, an L-type VOCC antagonist, on a range of autonomic reflexes in conscious rabbits. 2. Mean arterial pressure (MAP), heart rate (HR), the baroreceptor-HR reflex, postural adaptation reflex (90 degrees head-up tilt), Bezold-Jarisch-like reflex and the vasoconstrictor component of the nasopharyngeal reflex were assessed before and during i.v. infusion of vehicle (saline), mibefradil or verapamil. Doses of mibefradil that gave low (M1; 0.45 +/- 0.02 microg/mL) and high (M2; 0.93 +/- 0.05 microg/mL) plasma concentrations, or verapamil (0.059 +/- 0.004 microg/mL; n = 6 each) were chosen to mimic clinically observed therapeutic levels. 3. At steady state infusion over 30-90 min, MAP was significantly lower in M2 (- 7 mmHg) and verapamil (- 6 mm Hg) treatments, but only verapamil caused a significant tachycardia (+ 31 b.p.m.) compared with vehicle. Mibefradil (M2) and verapamil decreased the HR range of the baroreflex by 27 and 29%, respectively, but neither treatment affected the vagal or sympathetic constrictor components of the Bezold-Jarisch-like and nasopharyngeal reflexes, respectively. 4. In response to 90 degrees tilt, vehicle- and verapamil-treated rabbits responded with small rises in MAP of 4 +/- 2 and 8 +/- 2 mm Hg, respectively, 5 s into the upright posture, while M1 and M2 caused falls in MAP of 6 +/- 4 and 9 +/- 3 mm Hg, respectively, at 5 s. 5. Thus, both L- and T-type VOCC antagonists, at plasma concentrations in the clinical range, lowered MAP in the conscious rabbit, but only mibefradil caused postural hypotension. We conclude that T-type VOCC may play an important role in the venoconstrictor reflex in response to tilt in the rabbit.

Structural factors increase blood pressure through the interaction of resistance vessel geometry with neurohumoral and local factors: estimates in rabbits with renal cellophane-wrap hypertension with intact effectors and during neurohumoral blockade

BACKGROUND

The structural changes in hypertension include narrowing of the lumen of the large resistance vessels and an increase in their wall thickness : lumen ratio. Their haemodynamic role has been controversial.

OBJECTIVE

To examine resting haemodynamics and the responses to graded drug-induced changes in tone in renal cellophane-wrap (wrap) and sham-operated (sham) rabbits, when their neurohumoral effectors were intact and during high-level blockade.

METHODS

Each rabbit was implanted with a flow probe for measuring cardiac output, had catheters inserted for drug infusions, and underwent mean arterial pressure (MAP) measurements. Resting values were determined and we infused graded doses of dilator and constrictor drugs: acetylcholine or adenosine; angiotensin II or methoxamine. The dilator and constrictor dose-response curves were combined into a single relationship for MAP, cardiac output, total peripheral conductance (TPC) and heart rate; total peripheral resistance (TPR) was estimated as 1/TPC.

RESULTS

Throughout the range of vascular tone and with intact effector function, MAP was greater and TPC lower in wrap than in sham rabbits, and cardiac output was the same in both groups. The ratios of wrap : sham slopes of the log dose-response regression lines were 0.47 for TPC, 2.04 for TPR and 1.89 for MAP. Thus MAP and TPR responses were enhanced to the same degree in wrap rabbits. During neurohumoral block, baroreflex-mediated heart rate responses were abolished. In addition, resting vascular tone was lower than with intact effectors in both wrap and sham rabbits; however, the ratios of wrap : sham slopes for TPC and TPR were similar to those with intact effectors, whereas the ratio for the slope for MAP was slightly smaller, although still enhanced.

CONCLUSION

In wrap hypertension, the enhanced MAP and TPR responses are in accord with an interaction between vascular geometry and the sum of altered neurohumoral + local activity, plus a rarefaction component.

Cardiovascular and autonomic effects of omega-conotoxins MVIIA and CVID in conscious rabbits and isolated tissue assays

1. The effects of a novel N-type voltage-operated calcium channel antagonist, omega-conotoxin CVID, were compared with omega-conotoxin MVIIA on sympathetic-evoked activation of right atria (RA), small mesenteric arteries (MA) and vasa deferentia (VD) isolated from the rat. Their effects were also compared on blood pressure and cardiovascular reflexes in conscious rabbits. 2. The pIC(50) values for MVIIA and CVID, respectively, for inhibiting sympathetic-evoked responses were equivalent in RA (8.7 and 8.7) and VD (9.0 and 8.7); however, in MA the values were 8.4 and 7.7. The cardiac to vascular (RA/MA) potency ratios, antilog (plog RA - plog MA), for MVIIA and CVID were 2 and 10. The offset rates for CVID and MVIIA were rapid, and peptide reapplication caused rapid onset of blockade, suggesting limited desensitization. 3. In the conscious rabbit, CVID and MVIIA (100 microg kg(-1) i.v.) caused a similar fall in blood pressure and a tachycardia that rapidly reached maximum. Both peptides decreased the vagal- and sympathetic-mediated components of the baroreflex, but had no effect on the vagal nasopharyngeal reflex. The orthostatic reflex to 90 degrees tilt was blocked by MVIIA with sustained postural hypotension for > or = 90 min after administration. In contrast, CVID caused postural hypotension at 30 min which recovered rapidly. 4. Neither CVID nor MVIIA (3 microg kg(-1) i.t.) significantly altered cardiovascular variables or autonomic reflexes. 5. In conclusion, CVID appears to be relatively weak at inhibiting the reflex response to tilt consistent with its weaker inhibition of rat mesenteric artery constriction to perivascular nerve stimulation. This may point to subtype N-type calcium channel selectivity.

Techniques to measure pharmacodynamics in the intact vasculature

Techniques are described for the intravenous, close intraarterial, or perivascular delivery of drugs in conscious or anaesthetized animals. Examples of the determination of pharmacodynamic parameters such as regional blood flow, large artery diameter, resistance, conductance, and blood pressure are given for conscious rabbits and anaesthetized dog preparations. An important issue is how to determine the direct vascular action of an injected drug in the light of rapid and powerful autonomic reflex buffering effects especially in healthy conscious animals. The methods of measurement of drug action on the baroreceptor-heart rate reflex and postural adaptation (90 degrees tilt) reflex in the conscious rabbit are explained. Finally, the changes to large and small artery morphology are explored in the rabbit hindlimb following conduit femoral artery ligation to induce arteriogenesis and angiogenesis. This work aims to highlight approaches to exploring drug action in vivo, a much neglected skill in the repertoire of the modern cardiovascular pharmacologist.

Postural hypotension following N-type Ca2+ channel blockade is amplified in experimental hypertension

OBJECTIVE

To determine the relative importance of the cardiac and vascular sympathetic components of the orthostatic response to 90 degrees head-up tilt after N-type calcium-channel blockade in normotensive (sham renal cellophane wrap) and hypertensive (renal wrap) conscious rabbits.

METHODS

The effects of N-type calcium-channel blockade with omega-conotoxin GVIA (omega-CTX, 10 microg/kg i.v. bolus) were assessed in the absence or presence of cardiac block by propranolol and methscopolamine. These were contrasted with the effects of alpha1-adrenoceptor antagonism (prazosin 0.5 mg/kg i.v. bolus, in the presence of cardiac block) or ganglion blockade (mecamylamine 4 mg/kg i.v. bolus).

RESULTS

In vehicle (0.9% saline) treatment groups, the response to tilt consisted of a small pressor effect (4 +/- 2 and 7 +/- 1 mmHg) and tachycardia (29 +/- 6 and 17 +/- 6 beats/min) in sham (n = 6) and wrap (n = 5) rabbits, respectively. After prazosin administration (with cardiac block), there were significant falls in MAP of 3 +/- 1 and 7 +/- 2 mmHg in sham (n = 7) and wrap (n = 6) rabbits, respectively, in response to tilt omega-CTX caused postural hypotensive responses of 8 +/- 2 and 13 +/- 2 mmHg in sham (n = 6) and wrap (n = 7) rabbits, respectively, and 7 +/- 1 and 14 +/- 2 mmHg in sham (n = 7) and wrap (n = 7) rabbits with prior cardiac block. Similarly, mecamylamine caused falls in MAP of 8 +/- 1 and 10 +/- 2 mmHg in response to tilt in sham (n = 6) and wrap (n = 9) animals, respectively.

CONCLUSION

Sympathetic vasoconstrictor effectors are primarily responsible for maintaining blood pressure during tilt in conscious rabbits. The postural hypotension caused by sympatholytic agents is about double in hypertensive rabbits, and N-type calcium-channel blockade is as effective as ganglion blockade at inducing this syndrome.

Enhanced total peripheral vascular responsiveness in hypertension accords with the amplifier hypothesis

BACKGROUND

Increases in vascular resistance (or the reciprocal, vascular conductance) in response to constrictor drugs are amplified (or attenuated) in the hindquarter vascular bed of hypertensive rats and rabbits. However, such changes have not been observed for the total peripheral circulation.

OBJECTIVE

To assess whether the vascular amplifier applies to the total peripheral circulation in conscious hypertensive rabbits.

METHODS

Rabbits were implanted with a flow probe for measuring cardiac output, a left atrial catheter for infusing dilator (adenosine) and constrictor (methoxamine) drugs, and ear artery and vein catheters for measuring mean arterial pressure (MAP) and for giving ganglion blockade. Data from full dose-total peripheral resistance (TPR) or total peripheral conductance (TPC) response curves to adenosine and methoxamine were combined into a logistic function extending from near full dilatation to near maximum constriction.

RESULTS

Changes in MAP induced by methoxamine and adenosine were markedly greater in the 'wrap' rabbits (those with renal cellophane wrapping) compared with the sham animals. In the 'wrap' rabbits, the slopes and ranges of the adenosine-TPR response curve and the methoxamine-TPC response curve were 200% and 60%, respectively, of sham values. These data show that TPR changes are amplified, and TPC changes attenuated, to dilator and constrictor stimuli. The relationship between dose-average vascular radius (r; based on Poiseuille's law) over the full range of vascular tone showed that r was narrower in hypertension, but, in contrast to TPR, the degree of narrowing was almost the same between maximum dilatation and constriction.

CONCLUSION

The total peripheral circulation in experimental hypertension is a TPR amplifier, or TPC attenuator, in the rabbit, consistent with well-established data in the major vascular beds.