Ageing, hypertension and aortic valve stenosis - Understanding the series circuit using cardiac magnetic resonance and applanation tonometry

BACKGROUND

Aortic stenosis (AS) is no longer considered to be a disease of fixed left ventricular (LV) afterload, but rather, functions as a series circuit, with important contributions from both the valve and vasculature. Patients with AS are typically elderly, with hypertension and a markedly remodelled aorta. The arterial component is sizeable, and yet, quantifying this to-date has been difficult to determine. We compared measurement of aortic pressure, flow and global LV load using a cardiac magnetic resonance (CMR)/applanation tonometry (AT) technique to uncouple ventriculo-arterial (VA) interactions.

METHODS

20 healthy elderly patients and 20 with AS underwent a CMR/AT protocol. CMR provided LV volume and aortic flow simultaneously with AT pressure acquisition. Aortic pressure was derived by transformation of the AT waveform. Systemic vascular resistance (SVR) and global LV load were determined as the relationship of pressure to flow in the frequency domain. Values from both cohorts were compared.

RESULTS

AS patients were older (p ​< ​0.01) albeit with no significant difference in brachial or central aortic pressure. SVR (14228 vs 19906 ​dyne ​s.cm-3; p ​= ​0.02) and load (740 vs 946 ​dyne ​s.cm-3; p ​= ​0.02) were higher in patients with AS, whilst aortic peak flow velocity was lower (38 vs 58 ​cm/s; p ​< ​0.01).

CONCLUSIONS

Quantification of aortic pressure, flow velocity and global LV load using a simultaneous CMR/AT technique is able to demonstrate the progressive effects of hypertension and aortic stiffening with advanced age and valvular stenosis. This technique may help to better identify future patients at risk of VA coupling mismatch after correction of AS.

Augmented depression and reduced excitability of the central nervous system (CNS) by cadmium in the rat

Reports indicating that low doses of cadmium caused vasodilation, but that larger quantities elicited a pressor response, apparently mediated by a CNS reflex, prompted an examination of cadmium-induced changes in CNS responsiveness and activity. Rats were injected intraperitoneally with either 2 mg/kg or 4 mg/kg of CdCl2 solution, after which the CNS was either depressed by pentobarbital or excited by strychnine at different dose levels. Cadmium treatment, administered before pentobarbital, decreased the time required for sleep induction and prolonged sleep duration at doses of either 20 mg/kg or 30 mg/kg: at 40 mg/kg only induction was affected and at 60 mg/kg neither was influenced. At a dosage of 60 micrograms/kg, strychnine caused convulsions in all control animals, but in none pretreated with CdCl2. When either 75 or 120 micrograms/kg of strychnine was used, cadmium at either dosage failed to prevent convulsions, although the onset was delayed and duration curtailed. The rapidity with which Cd modified CNS activity indicated that the effect can not depend upon cadmium-induced synthesis of metallothionine, but represents a direct effect of Cd on the CNS. Cadmium treatment did not substantially improve the survival of rats that convulsed when treated with strychnine.

Control of spontaneous and deoxycorticosterone-salt hypertension and polyuria by nitrendipine pellets

Weekly subcutaneous implantation of 25-mg nitrendipine pellets prevented onset of both spontaneous and deoxycorticosterone-salt hypertension in rats. Discontinuance of implantation led to reappearance of hypertension after about 2 weeks in the former and led to rising though still normotensive pressures after about 3 weeks in the latter. A new implant caused blood pressures in both to drop within a day or two to normotensive levels in the case of spontaneously hypertensive rats. Nitrendipine prevented cardiac hypertrophy in steroid hypertensive rats, but not in spontaneous hypertensives. A nitrendipine pellet given 1 day before or a 30 mg/kg injection given 1 h prior to the administration of a water, Na+, and K+ load, prevented the diabetes insipidus-like syndrome resulting from deoxycorticosterone-salt treatment, and lowered sodium but not potassium excretion. Nitrendipine did not affect steroid-induced hypernatremia and hypokalemia.

Prevention of spontaneous and glucocorticoid hypertension in rats by nisoldipine

The effect of daily treatment (sometimes interrupted) with the calcium blocker nisoldipine at two different dose levels, one double the other, on the development of spontaneous hypertension in the rat was evaluated. Both doses prevented development of the disorder during daily administration, but when treatment was stopped the same degree of hypertension as in controls quickly developed. Resumption of treatment caused blood pressure to fall to or towards normal again. The higher dose caused a more rapid and greater fall than the lower, and allowed a lesser pressor response when it was discontinued. The drug had no effect on growth, and the only consistent hematological effect was a slight thrombocytosis. Daily treatment at a single dose level slightly reduced normal blood pressure and prevented cortisone hypertension. Serum renin activity was unaffected by nisoldipine but was elevated by cortisone treatment: nisoldipine increased aldosterone levels, but not when cortisone was also given.

Structural, functional and hypertensive effects of 19-oxo-11-deoxycorticosterone acetate (19-oxo-DOCA) in the rat

Mononephrectomized rats were given 1% NaCl solution to drink; half of them received 1 mg/day of 19-oxo-11 deoxycorticosterone acetate (19-oxo-DOCA) in sesame oil subcutaneously and half received only the oil for a period of four weeks. The steroid had no effect upon saline intake, systolic blood pressure, growth or the size of adrenals, hearts or kidneys, although it did produce hypernatremia and hypokalemia. The discrepancy between a demonstrable mineralocorticoid effect without blood pressure elevation awaits elucidation.

Prevention of DOCA-salt hypertension with the calcium blocker nitrendipine

Mononephrectomized female rats on a high sodium intake developed hypertension, hypokalemia, enlarged hearts and kidneys and slight adrenal involution under deoxycorticosterone treatment. Simultaneous administration of nitrendipine (5 mg/kg twice daily) completely prevented hypertension and reduced but did not abolish cardiac enlargement. There was no effect of the calcium slow-channel inhibitor on kidney enlargement, adrenal atrophy or hypokalemia. The ability of the steroid to produce cardiomegaly in the absence of an elevated blood pressure to account for it, tends to confirm the suggestion of other investigators that the steroid may have that effect by a mechanism not involving blood pressure elevation.

Influence of dosage, consciousness, and nifedipine on the acute pressor response to intraperitoneally administered cadmium

The acute pressor effect of intraperitoneally administered cadmium was explored over the dose range 0.015-2 mg/kg in both pentobarbital-anesthetized and conscious rats. The former first respondent at 0.031 mg/kg, and successive doublings of that dosage increased the highest pressures attained in a stepwise fashion until a dosage of 0.25 mg/kg, the maximally effective quantity, was reached. Arterial pressure did not rise in conscious rats until a dose of 1 mg/kg, which gave the maximum response within the range examined. Heart-rate changes with Cd were slight, and rarely significant at a given dosage, but pentobarbital invariably caused tachycardia. Anesthetized rats thus gave a graded response, while conscious animals reacted in an all-or-none fashion. The increased pressor responsiveness of rats under pentobarbital can not be ascribed to its cardiac parasympatholytic effects, since sensitivity was not conferred upon conscious rats when pretreated with atropine at a dose producing even greater tachycardia than that caused by pentobarbital. Nifedipine, which blocks calcium entry into smooth muscle cells, prevented the pressor response to cadmium when given as pretreatment and terminated an ongoing response when give intercurrently. Possible mechanisms to account for the observed behavior are considered.

Similarities and differences between effects of testosterone and 19-nortestosterone in rats, with particular reference to hypertensogenic potency

Both 19-nordeoxycorticosterone and 19-norprogesterone are potent hypertensogens. This particularly interesting in the latter case, since the parent steroid is antimineralocorticoid and antihypertensive. The present experiment compared the ability of testosterone and 19-nortestosterone to cause hypertension in rats. Both steriods caused adrenal atrophy, nephromegaly, relative hypoproteinemia and increased hematocrit, but only testosterone provoked saline polydipsia, hypernatremia, hypertension, cardiomegaly and vascular lesions. It is evident that demethylation of testosterone at C10 completely destroys any effect on sodium metabolism or blood pressure, but leaves certain other pathophysiologic responses, including extreme adrenal atrophy, unimpaired. The hypertensogenic effect of testosterone has been attributed to its inhibitory effect on adrenal structure and function, the latter characterized by an induced enzymatic defect leading to increased secretion of deoxycorticosterone. This raises the intriguing question of whether, despite the comparable involution of the adrenal cortex, there are significant differences in adrenocortical enzymatic changes initiated by the respective androgens, which could account for their quiet different blood pressure effects.

Inhibition of DOC-salt and adrenal-regeneration hypertension with the calcium blocker nifedipine

When nifedipine was given to rats at a dosage of 0.3 mg/kg, the blood pressure fell and heart rates rose slightly in normotensive controls, but both changes were more marked in rats with DOC-salt hypertension, the magnitude of the drop in mm Hg being progressively greater as hypertension evolved. Tachycardia also tended to increase, but became somewhat erratic once hypertension was established. Both changes were relatively constant and equivalent when expressed as a percentage of the initial value. Nifedipine given by minipump at a dosage of 8 mg/ml proved able to prevent or modulate, adrenal-regeneration hypertension, but not DOC-salt hypertension. However, in the latter, when the pumps were discontinued and a 1 mg/kg/day injection schedule was substituted, blood pressure fell in nifedipine-treated and rose in untreated rats. The latter dosage had a greater effect on blood pressure and heart rate of normotensive rats than 0.3 mg/kg.

Mineralocorticoid and hypertensive effects of 19-nor-progesterone

The mineralocorticoid potency of 19-nor-progesterone was evaluated by both its effect on electrolyte excretion in adrenalectomized animals and its ability to cause hypertension and electrolyte changes in mononephrectomized, salt-loaded rats. The mineralocorticoid activity, measured using an adrenalectomized rat bioassay, indicated that 19-nor-progesterone was 2.5% as potent as aldosterone but did not antagonize the effect of aldosterone when both were administered. In mononephrectomized rats, the daily administration of 1 mg/day quickly caused an enhanced consumption of 1% saline and induced severe hypertension within 3-4 weeks. Some severely hypertensive animals had marked anemia, but other did not; as a group they were found to have hypernatremia and hypokalemia. Hypertensive animals were found during life to display a relative hypothermia and, at necropsy, to have heart and kidney enlargement with severe and extensive vascular lesions in both organs, but not adrenal hypertrophy. It is concluded that 19-nor-progesterone has the characteristics of a potent mineralocorticoid and, as such, is capable of causing hypertension. It is not yet clear why this should be accompanied by hypothermia.