Sympathetic activity and cardiac adrenergic receptors in one-kidney, one clip hypertension in rats

Effects of pre-existing left ventricular hypertrophy on ventricular dysfunction and remodeling following myocardial infarction in rats

BACKGROUND

Myocardial hypertrophy is a characteristic component of left ventricular (LV) remodeling that may, at least initially, have a beneficial effect on LV function following myocardial infarction (MI). In the present study, we examine the effects of pre-existing left ventricular hypertrophy (LVH) on LV function and chamber enlargement following MI in inbred Lewis rats.

METHODS

The one-kidney, one-clip model (1K1C) of hypertension was used to produce LVH. Four weeks after 1K1C, rats were randomized to left anterior descending coronary artery ligation (LVH + MI group, n = 8) or sham ligation (LVH group, n = 11). Another group of rats underwent sham 1K1C. Four weeks later, they were randomized to coronary ligation (MI group, n = 12) or sham ligation (Sham group, n = 12). LV end-diastolic pressure (EDP, mm Hg), end-diastolic volume (EDV, ml), end-systolic volume (ESV, ml) and ejection fraction (EF) (determined by angiography) were measured in all groups 2 months after MI.

RESULTS

LV EDP was 20 +/- 2 mm Hg in the LVH + MI group compared with 9 +/- 1 mm Hg in the MI group (p < 0.05). LV EDV and ESV were significantly greater with LVH + MI than with MI alone (EDV 0.90 +/- 0.03 vs 0.75 +/- 0.02 ml; ESV 0.68 +/- 0.02 vs 0.50 +/- 0.03 ml; p < 0.05). Pre-existing LVH resulted in a greater reduction in EF following MI (25 +/- 2% for LVH + MI vs 34 +/- 2% for MI alone; p < 0.05).

CONCLUSIONS

Pre-existing LVH is an important determinant of progressive LV dysfunction and remodeling following MI in Lewis inbred rats.

Norepinephrine turnover in the heart and blood vessels of Goldblatt hypertensive rats

The norepinephrine (NE) content, the uptake of [3H]NE, the turnover time, the turnover and the synthesis rate of the neurotransmitter in the heart and blood vessels were studied in the chronic phase of two kidney and one kidney Goldblatt renovascular hypertension, in the rat. Intact and sham operated animals were used as controls. Fifty percent of the rats subjected to renal clipping developed hypertension. This fact allowed us to compare changes in clip operated hypertensive and normotensive animals. The weight of the hearts and blood vessels was significantly increased in clip operated rats. Changes were greater in hypertensive animals. NE concentration and total content in the heart and in the artery wall were significantly decreased in the clipped rats. [3H]NE uptake was significantly diminished in the heart of experimental animals; in the artery wall, uptake was much lower than in the heart but no differences were observed between clip operated and sham animals. The turnover of NE was not different among control and clip operated rats either in the heart or in the blood vessels. Synthesis rate was lower in hypertensive animals than in their respective controls, explaining the lower concentration of the amine in cardiovascular tissues. The present data do not suggest that an increased turnover of NE in the cardiovascular sympathetic nerve endings is involved in the maintenance of high blood pressure in both types of Goldblatt renovascular hypertension.

Potentiation of reperfusion-associated ventricular fibrillation by left ventricular hypertrophy

Important electrophysiological alterations that may predispose hearts to arrhythmias have been described for hypertrophied myocytes, and hypertrophy coupled with ischemia has been associated with an increased incidence of sudden death; however, an influence of hypertrophy on reperfusion arrhythmias has not been previously described. We hypothesized that reperfusion-associated arrhythmias would be potentiated by left ventricular hypertrophy. After induction of renovascular hypertension, 37 awake, unsedated dogs (17 with left ventricular hypertrophy and 20 without hypertrophy) underwent 15 minutes of coronary artery occlusion and reperfusion. All dogs were pretreated with lidocaine bolus injections and with lidocaine by continuous infusion during coronary occlusion and reperfusion. Reperfusion-associated ventricular fibrillation occurred in seven of 17 dogs with left ventricular hypertrophy versus one of 18 dogs without hypertrophy (p less than or equal to 0.05). The presence of hypertension was not significantly associated with an increased incidence of reflow ventricular arrhythmias. Neither QT interval nor area-at-risk was different between the dogs with and without reperfusion ventricular fibrillation; however, increased heart rate just before reperfusion did correlate with an increased incidence of ventricular fibrillation at reperfusion. Thus, 1) left ventricular hypertrophy was associated with a significantly increased incidence of reperfusion-induced ventricular fibrillation after 15 minutes of ischemia, 2) this increased incidence was independent of the presence of hypertension, and 3) lidocaine protected control and hypertrophied hearts against ventricular fibrillation during ischemia but was ineffective in protecting hypertrophied hearts against reperfusion-induced ventricular fibrillation.

Goldblatt hypertension and operant thermoregulation in shaved, sialoadenectomized rats

The thermobehavioral consequences of angiotensinogenic two-kidney (2K) and sodium-volume dependent one-kidney (1K) forms of Goldblatt (one-clip) renovascular hypertension were assessed in shaved, sialoadenectomized rats. The 2K group (n = 8) underwent unilateral renal artery stenosis with the contralateral kidney left intact; whereas the Sham-Operation (Sham-Op) Control group (n = 7) received only a laparotomy. The 1K group (n = 8) underwent unilateral renal artery stenosis with contralateral nephrectomy, and the Uninephrectomy Control group (n = 6) was only unilaterally nephrectomized. Shaping and testing was conducted in a convective thermal controller that permitted the experimenters to control, through continuously reinforced bar-pressing, the ambient temperature during exposure to warm (37 degrees C) or cold (17 degrees C) temperatures. Testing at each temperature occurred over two 6-hr sessions with an interpolated 48-hr rest period. Both 2K and 1K hypertensive rats exhibited longer durations of heat escape than their appropriate normotensive controls across both testing sessions. No differences were detected in response frequency, duration of cold escape responding, or body temperature. The results suggest that either the increased vascular resistance or the neuroendocrine-sympathetic disorder linked to the pathogenesis of Goldblatt renovascular hypertension may promote heat retention or lower heat tolerance.

Blood ethanol level and physiologic measurements during ethanol-induced hypertension

Plasma norepinephrine levels and alpha 2-adrenoceptor binding in central nervous system areas involved in blood pressure regulation were determined during the development of ethanol-induced hypertension in rats. Blood pressure was increased after the 4th week of ethanol feeding. Plasma norepinephrine levels of the ethanol-fed group were lower than those of the control group at most time periods studied even though blood pressure was elevated. 3H-Clonidine binding to alpha 2-adrenoceptors was decreased in the anterior and posterior hypothalamus of ethanol-fed rats throughout most of the study. Blood pressure in the ethanol-fed group was elevated when blood ethanol concentration was high, whereas blood pressure in this group was similar to that of the control group when blood ethanol concentration was low. Brain and plasma samples were taken during low blood ethanol which may explain the lack of positive findings. These results suggest that ethanol is a pressor agent and must be present to exert its cardiovascular effects. These findings suggest the importance of blood ethanol concentration at the time of study and also suggest that the blood pressure elevating effects of ethanol may be central rather than peripheral in origin.

Beta-adrenoceptor and adenylate cyclase regulation in cardiac myocyte growth

We studied the effect of growth on beta-adrenergic receptor properties of neonatal rat heart myocytes cultured in serum-free medium with transferrin and insulin. Growth was induced by addition of 1 microM (-)-norepinephrine for two days, 200 nM of the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) for two days, or 30 nM T3 for six days. The Kd values for beta-receptor binding (125I-ICYP) were unaffected by growth. The maximum number of beta-receptor binding sites calculated as sites/cell was increased 1.47-fold by T3 (p less than .005), but was decreased to 54% of control values by (-)-norepinephrine (p less than .005): TPA had no effect on either Kd or Bmax values. (-)-Isoproterenol-stimulated adenylate cyclase activity was augmented only in membranes from T3-treated cells and was reduced by 69% in membranes from (-)-norepinephrine treated cells. TPA had no effect on (-)-isoproterenol-stimulated adenylate cyclase activity. We conclude that the mechanisms controlling beta-adrenergic receptor number may be distinct from those controlling growth, since receptor number does not correlate with cell enlargement. Furthermore, in (-)-norepinephrine-stimulated growth, which we have shown previously is an alpha 1-adrenoceptor mediated response, beta-adrenergic signal transduction is modulated in a directionally opposite fashion.

Adrenal and vascular tyrosine hydroxylase activity in Goldblatt hypertension

To examine the role of the sympathetic nervous system in hypertension, the in vitro activity of tyrosine hydroxylase was examined in one-kidney, one clip (1K1C) and two-kidney, one clip (2K1C) hypertensive rabbits and their respective controls 2 weeks after surgical procedures. The in vitro activity of tyrosine hydroxylase provides a measure of catecholamine synthesis and serves as a biochemical index of activity of noradrenergic neurons and the adrenal medulla. Mean atrial pressure rose from 91.5 +/- 1.0 to 128.5 +/- 5.6 mm Hg (p less than 0.01) in the 1K1C group and from 91.8 +/- 1.3 to 106.5 +/- 5.0 mm Hg (p less than 0.02) in the 2K1C group, whereas no change in blood pressure was found in their respective controls. Adrenal tyrosine hydroxylase activity was increased 85% in the 1K1C group, as compared with values in one-kidney controls (from 11.8 +/- 1.5 to 21.8 +/- 1.1 pmol CO2/min/mg; p less than 0.0002), and was increased 49% in the 2K1C group, as compared with values in two-kidney controls (from 8.01 +/- 1.2 to 11.9 +/- 1.1 pmol CO2/min/mg; p less than 0.02). In the 1K1C group, proximal mesenteric tyrosine hydroxylase activity was decreased 46% compared with values in one-kidney controls (from 23.5 +/- 5.0 to 12.8 +/- 2.5 pmol CO2/min/mg; p less than 0.03) and distal mesenteric tyrosine hydroxylase activity was decreased 42% (from 7.73 +/- 1.2 to 4.46 +/- 0.8 pmol CO2/min/mg; p less than 0.03). In the 2K1C group, neither proximal nor distal mesenteric tyrosine hydroxylase activity was altered. Tyrosine hydroxylase activity was not detectable in the femoral arteries, or in the thoracic and abdominal aorta.(ABSTRACT TRUNCATED AT 250 WORDS)