Influence of chronic nadolol treatment on blood pressure and vascular changes in spontaneously hypertensive rats.

Chronic treatment of spontaneously hypertensive rats (SHR) and Kyoto-Wistar normotensive rats (WKY) with nadolol was carried out from gestation until 28 weeks of age. Nadolol treatment caused some lowering of blood pressure but did not prevent the development of hypertension or cardiac hypertrophy in the SHR, in spite of significant beta-blockade. The lumen of large mesenteric arteries from control SHR was smaller than from WKY, and nadolol treatment increased the lumen size in the SHR. An increased number of smooth muscle cell layers present in the control SHR as compared with WKY was reduced slightly by nadolol treatment. However, the changes produced by nadolol did not reach the levels of control and treated WKY. In the aorta, the incidence of polyploid smooth muscle cells was higher in the SHR than the WKY in the control group. Nadolol treatment reduced the percentage of polyploid cells in both SHR and WKY, so that the difference between these two groups of animals was eliminated in the treated groups. The tissue level of norepinephrine in the plasma, heart, mesenteric arteries, and adrenal glands in the SHR and WKY was not affected by the treatment. We suggest that the ineffectiveness of nadolol in preventing hypertension development may be due to its lack of effect in preventing primary changes in the resistance arteries, and that the development of polyploidy of smooth muscle cells may be mediated by beta-receptors.     

Increased sympathetic innervation in the cerebral and mesenteric arteries of hypertensive rats

The density of catecholamine-containing nerve fibers was studied in the cerebral and mesenteric arteries from normotensive Wistar-Kyoto rats (WKY), spontaneously hypertensive rats (SHR), and stroke-prone SHR (SHRSP) in the growing (SHR, WKY) and adult (SHR, SHRSP, WKY) animals. Cerebral arteries from SHR showed an increased adrenergic innervation from day 1. The nerve plexuses reached an adult pattern earlier in SHR than in WKY. The arteries from adult SHR and SHRSP (22 weeks old) showed a markedly higher nerve density than WKY. There was a positive linear correlation between blood pressure and nerve density for four cerebral arteries. The mesenteric arteries were not innervated at birth. However, hyperinnervation of these arteries in the SHR was already present at 10 days of age as compared with WKY. Sympathectomy with anti-nerve growth factor and guanethidine caused a complete disappearance of fluorescent fibers in the mesenteric arteries from SHR and WKY, and in the cerebral arteries of WKY. The same procedure caused only partial denervation of the cerebral arteries from hypertensive animals. We postulate that the increase in nerve density in the cerebral arteries from the hypertensive rats may contribute to the development of arterial hypertrophy in chronic hypertension through the trophic effect of the sympathetic innervation on vascular structure.     

Prolonged isobaric relaxation time in small mesenteric arteries of the spontaneously hypertensive rat

Prolonged isometric relaxation in hypertensive aortic and caudal arterial smooth muscle has been demonstrated; however, isobaric relaxation in resistance arteries is more pertinent to studies in hypertension. A comparative study of mesenteric arterial isobaric relaxation times was made using spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto rats (WKY), and MK-421 treated SHR (treatment commenced at 8 weeks of age and was maintained until sacrifice). Relaxation rates of vessels constricting against a range of pressures and achieving different degrees of narrowing or changes in circumference were analyzed. Comparisons were made between SHR, WKY, and MK-421 treated SHR arteries that had constricted from the same initial circumference and against the same magnitude of pressure. The SHR mesenteric arteries relaxed at a slower rate than did the WKY vessels. The normotensive MK-421 treated SHR showed the same prolonged relaxation rate as did the untreated SHR preparations. Thus the slower rate of relaxation in SHR arteries does not appear to be a consequence of the hypertension. Such prolonged time for narrowing would function to increase the average peripheral resistance and thus may contribute to the initiation and maintenance of increased blood pressure.     

The relationship between altered blood vessel structure, hypertension, and the sympathetic nervous system

The relationship between sympathetic innervation and arterial medial development has been examined in normotensive, hypertensive, and diabetic rats. Using the jejunal artery as a model, the number of nerve fibres innervating the artery as determined from fluorescent preparations, and the medial thickness and lumen diameter as measured from resin embedded specimens were correlated from animals prepared in various ways. The rats used were normal Sprague-Dawley (SD), SD with induced hypertension, SD with diabetes induced with streptozotocin, SD sympathectomized with 6-hydroxydopamine, spontaneously hypertensive rats (SHR), SHR treated with capsaicin to prevent hypertension development, Wistar Kyoto rats (WKY), and WKY treated with capsaicin. Examination of the jejunal arteries from these rats at 12 weeks of age following normal development, or 8 weeks of hypertension development, or 8 and 12 weeks of diabetes, showed that increased innervation occurred in the SHR under all conditions, and in the diabetic rats after 8 weeks of diabetes. Medial hypertrophy occurred in the SHR and in the SD hypertensive only. It is concluded that the special relationship which exists between the sympathetic innervation and arterial media in the SHR does not occur during hypertension development in the SD rat, nor is it necessary for normal medial development in the SD rat. The sympathetic innervation does appear to have a trophic influence on vascular smooth muscle of diabetic rats, at least in the early stages of the disease.     

Dependence of the number of vesicles in vascular smooth muscle cells on the presence of noradrenaline in the extracellular space

Noradrenaline (NA) effect on the number of vesicles in smooth muscle cells was investigated in small mesenteric arteries of spontaneously hypertensive rats (SHR), aged 8 or 12 weeks, and age-matched normotensive Wistar-Kyoto (WKY) rats. The presence of NA in the incubation medium resulted in an increase in the number of vesicles in SHR of both age groups, but not in WKY. The results are discussed in view of the relationship between the vesicles and Ca transport in smooth muscle cells.     

Incidence of multinucleated and polyploid aortic smooth muscle cells cultured from different age groups of spontaneously hypertensive rats.

Cell size and incidence of multinucleated, polyploid cells in cultured aortic smooth muscle cells from different age groups of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) were compared. Smooth muscle cells from SHR were generally larger than those from WKY, and the percentage of multinucleated smooth muscle cells was always higher in SHR than WKY in the three age groups of rats studied (3-4, 10-12, and 28-30 weeks). In smooth muscle cells from the 3- to 4-week group, there was a positive correlation between cell diameter and the percentage of multinucleated smooth muscle cells. Microdensitometric measurements also showed that the incidence of polyploid smooth muscle cells was always higher in SHR than WKY in the three age groups. There was a positive correlation between DNA density and nuclear area measurements in all the age groups of SHR and WKY. We conclude that cultured aortic smooth muscle cells from different age groups of SHR and WKY contained heterogeneous populations of cells and that, under our culture conditions, the polyploidy of the smooth muscle cells found in vivo was maintained in the SHR and WKY.     

Changes of beta-adrenoceptors in the aortic muscle cells of spontaneously hypertensive rats

The characteristics of [125I]monoiodocyanopindolol (ICYP) binding to beta-adrenoceptors of cultured aortic smooth muscle cells derived from 4-week-old spontaneously hypertensive rats (SHR) and the Wistar-Kyoto normotensive rats (WKY) were examined. During optimization of the binding assays, we found that the specific binding of ICYP by intact cells was masked by a high level of nonspecific ICYP accumulation in intact cells presumably owing to the lipophilic nature of ICYP. Optimal specific ICYP binding requires that the cells be gently lysed with hypotonic dilution followed by a freeze-and-thaw cycle. Under most experimental conditions tested, the total number of ICYP binding sites in WKY aortic muscle cells was considerably and consistently smaller than that in SHR cells. There was no difference in the Kd values for ICYP binding to SHR and WKY cells. However, when ICYP binding was carried out using crude membrane fractions with well-defined plasma membrane content isolated from aortic muscle strips of adult rats, we found no difference in the number of beta-adrenoceptor sites between SHR and WKY. Morphological evidence indicated that cultured SHR aortic muscle cells contained a greater proportion of larger cells with multinuclear features. These results suggest that an increase in the number of beta-adrenoceptor density per cell in SHR may be associated with cellular hypertrophy of aortic smooth muscle cells. We conclude that under cultured conditions, a higher incidence of polyploid smooth muscle cells in the SHR as compared with WKY was expressed earlier than under in vivo conditions. Therefore, the interpretation of results obtained from cultured cell studies in relation to under in vivo conditions should be exercised with caution.     

Relative contribution of Rho kinase and protein kinase C to myogenic tone in rat cerebral arteries in hypertension

Arterial smooth muscle constriction in response to pressure, i.e., myogenic tone, may involve calcium-dependent and calcium-sensitization mechanisms. Calcium sensitization in vascular smooth muscle is regulated by kinases such as PKC and Rho kinase, and activity of these kinases is known to be altered in cardiovascular disorders. In the present study, we evaluated the relative contribution of PKC and Rho kinase to myogenic tone in cerebral arteries in hypertension. Myogenic tone and arterial wall calcium in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) were measured simultaneously, and the effect of PKC and Rho kinase inhibitors on myogenic tone was evaluated. SHR arteries showed significantly greater myogenic tone than WKY arteries. Pressure/wall tension-arterial wall calcium curves showed a hyperbolic relation in WKY rats, but the curves for SHR arteries were parabolic. Myogenic tone was decreased by the Rho kinase inhibitors Y-27632 and HA-1077, with a significantly greater effect in SHR than in WKY arteries. Reduction in myogenic tone produced by the PKC inhibitor bisindolylmaleimide I in WKY and SHR arteries was significantly less than that produced by Rho kinase inhibition. The pressure-dependent increase in myogenic tone was significantly decreased by Y-27632, and the decrease was markedly greater than that produced by bisindolylmaleimide I in SHR arteries. In WKY arteries, the pressure-dependent increase in myogenic tone was decreased to a similar extent by Y-27632 and bisindolylmaleimide I. These results suggest greater myogenic tone with increased calcium sensitization in SHR arteries, largely because of Rho kinase activation, with a minor contribution of PKC activation.     

Decreased velocity of shortening in arterial smooth muscle from older (28- to 31-week-old) spontaneously hypertensive rats

An increased maximum velocity of shortening (Vmax) and increased shortening ability (delta Lmax) have been reported for caudal arterial smooth muscle from 16- to 18-week-old spontaneously hypertensive rats (SHR) compared with age-matched Wistar-Kyoto (WKY) control rats. It is known that hypertension results in hypertrophy of vascular smooth muscle. It is plausible that the faster Vmax of 16- to 18-week-old SHR arterial smooth muscle may slow down with age due to hypertrophy. The force-velocity (F-V) study done previously on caudal arterial strips from 16- to 18-week-old SHR and WKY rats was repeated on preparations from 28- to 31-week-old rats. An electromagnetic muscle lever was employed in recording force-velocity data. Analysis of these data revealed that the 28- to 31-week-old SHR (n = 7) mean F-V curve was not different from the 28- to 31-week-old WKY (n = 5) mean F-V curve (p greater than 0.05), and the shortening ability of 28- to 31-week-old SHR arterial muscle was significantly depressed compared with 28- to 31-week-old WKY arterial muscle (p less than 0.01). In conclusion, (i) although Vmax is faster in younger (16- to 18-week-old) SHR compared with age-matched WKY caudal arterial smooth muscle, SHR Vmax is not different from WKY Vmax in the older (28- to 31-week-old) rats. (ii) Shortening ability is greater in 16- to 18-week-old SHR caudal arterial strips compared with 16- to 18-week-old WKY strips, but is significantly depressed in 28- to 31-week-old SHR compared with 28- to 31-week-old WKY preparations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Age-dependent ultrastructural changes of coronary artery in spontaneously hypertensive rats

The age-dependent differences in basic cardiovascular parameters, geometry and structure of coronary arteries between Wistar and spontaneously hypertensive rats (SHR) were evaluated. SHR of the age 3-, 9-, 17-, and 52-week and age-matched Wistar rats were used. Blood pressure (BP) was measured by the plethysmographic method. Animals were perfused with a glutaraldehyde fixative under pressure of 90 mmHg (3-week-old) and 120 mmHg (9-, 17-, 52-week-old). Coronary arteries were processed for electron microscopy. The proportions and cross sectional areas (CSA) of extracellular matrix in intima and media, endothelial and muscle cells were determined by point counting method. Cardiac hypertrophy and except of 3-week-old rats also BP increase and coronary wall hypertrophy was found in all ontogenic periods in SHR compared to Wistar rats. Arterial wall hypertrophy was evoked by increase of CSA of medial extracellular matrix and smooth muscle cells. In 52-week-old SHR, CSA of muscle cells did not differ from that in 17-week-old SHR but the CSA of intimal and medial extracellular matrix significantly increased. The CSA of endothelial cells and CSA of intimal extracellular matrix were increased only in 52-week-old SHR. The independency between BP and trophicity of individual components of the coronary wall during ontogeny of SHR was documented.     

Studies of arterial smooth muscle relaxation in younger (16-18 week) and older (28-31 week) spontaneously hypertensive rats

Both isometric and isotonic relaxation rates have previously been reported to be decreased in caudal arterial and mesenteric resistance arterial smooth muscle from 16- to 21-week-old spontaneously hypertensive rats (SHR) compared with muscle from age-matched normotensive Wistar-Kyoto rats (WKY). An increased maximum velocity of shortening (Vmax) and an increased shortening ability (delta Lmax) have also been reported for arterial smooth muscle from 16- to 21-week-old SHR. It has been suggested that both increased narrowing and prolonged narrowing of arteries contribute to the development of hypertension. However, SHR Vmax is not different from WKY Vmax when studying arterial muscle from older (28- to 31-week-old) rats. Thus increased arterial narrowing ability cannot be a contributing factor to the maintenance of hypertension. In this study the role of relaxation rate in the maintenance of hypertension was examined by comparing the relaxation rates of isometric and isotonic contractions of caudal arterial strips from 16- to 21-week-old SHR (n = 9) and WKY (n = 8) and from 28- to 31-week-old SHR (n = 7) and WKY (n = 5). While relaxation rates were lower for 16- to 21-week-old SHR compared with age-matched WKY preparations for both isometric and isotonic contractions, only isometric relaxation rates were found to be different in 28- to 31-week-old SHR compared with 28- to 31-week-old caudal arterial muscle (p less than 0.05). Vmax tended to normalize from a once-elevated velocity, while isometric relaxation rate remained decreased in SHR with ageing and (or) with progression of the hypertensive condition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Liver growth factor treatment restores cell-extracellular matrix balance in resistance arteries and improves left ventricular hypertrophy in SHR

Liver growth factor (LGF) is an endogenous albumin-bilirubin complex with antihypertensive effects in spontaneously hypertensive rats (SHR). We assessed the actions of LGF treatment on SHR mesenteric resistance and intramyocardial arteries (MRA and IMA, respectively), heart, and vascular smooth muscle cells (VSMC). SHR and Wistar-Kyoto (WKY) rats treated with vehicle or LGF (4.5 μg LGF/rat, 4 ip injections over 12 days) were used. Intra-arterial blood pressure was measured in anesthetized rats. The heart was weighted and paraffin-embedded. Proliferation, ploidy, and fibronectin deposition were studied in carotid artery-derived VSMC by immunocytochemistry. In MRA, we assessed: 1) geometry and mechanics by pressure myography; 2) function by wire myography; 3) collagen by sirius red staining and polarized light microscopy, and 4) elastin, cell density, nitric oxide (NO), and superoxide anion by confocal microscopy. Heart sections were used to assess cell density and collagen content in IMA. Left ventricular hypertrophy (LVH) regression was assessed by echocardiography. LGF reduced blood pressure only in SHR. LGF in vitro or as treatment normalized the alterations in proliferation and fibronectin in SHR-derived VSMC with no effect on WKY cells. In MRA, LGF treatment normalized collagen, elastin, and VSMC content and passive mechanical properties. In addition, it improved NO availability through reduction of superoxide anion. In IMA, LGF treatment normalized perivascular collagen and VSMC density, improving the wall-to-lumen ratio. Paired experiments demonstrated a partial regression of SHR LVH by LGF treatment. The effective cardiovascular antifibrotic and regenerative actions of LGF support its potential in the treatment of hypertension and its complications.     

Myocardial hypertrophy of normotensive Wistar-Kyoto rats

In our studies with spontaneously hypertensive (SHR), Wistar-Kyoto (WKY), and Wistar rats, we observed normotensive WKY rats with cardiac hypertrophy determined by a greater left ventricular (LV) mass (LVM)-to-body weight (BW) ratio (LVM/BW) than that of normotensive Wistar rats. Thus we compared the following parameters in SHR, WKY, and Wistar rats: LVM/BW, cell capacitance as index of total surface area of the myocytes, length, width, and cross-sectional area of cardiac myocytes, LV collagen volume fraction, and myocardial stiffness. The LVM/BW of WKY (2.41 +/- 0.03 mg/g, n = 41) was intermediate between SHR (2.82 +/- 0.04 mg/g, n = 47) and Wistar rats (1.98 +/- 0.04 mg/g, n = 28). A positive correlation between blood pressure and LVM was found in SHR, whereas no such relationship was observed in WKY or Wistar rats. Cell capacitance and cross-sectional area were not significantly different in SHR and WKY rats; these values were significantly higher than those of Wistar rats. The cell length was smaller but the width was similar in WKY compared with SHR. Papillary muscles isolated from the LV of WKY and SHR were stiffer than those from Wistar rats. Consistently, a greater level of myocardial fibrosis was detected in WKY and SHR compared with Wistar rats. These findings demonstrate blood pressure-independent cardiac hypertrophy in normotensive WKY rats.     

1,25 (OH)2 vitamin D3-induced 45CA uptake in vascular myocytes cultured from spontaneously hypertensive and normotensive rats

The effect of 1,25 (OH)2 vitamin D3 on basal 45Ca uptake was examined in vascular smooth muscle cells cultured from mesenteric arteries of spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) normotensive rats. Basal uptake of 45Ca was significantly greater in myocytes of WKY than SHR at 5, 10, 30 and 60 min incubation with the isotope. Incubation with 1 ng/ml 1,25 (OH)2 vitamin D3 for 48 hr increased basal 45Ca uptake between 1-10 min in SHR and between 5-10 min in WKY. The dose-response relationship indicated that cells from both strains are equally sensitive to the calciotropic effects of 1,25 (OH)2 vitamin D3 with half-maximal stimulation occurring at approximately 0.3-0.4 ng/ml. In cells of both strains maximal stimulation of 45Ca uptake was achieved only after a 12-24 hr period of incubation with hormone and pretreatment with cycloheximide inhibited 1,25 (OH)2 vitamin D3-enhanced 45Ca uptake. Although 45Ca binding by extracellular matrix material was significantly greater in WKY than SHR, 1,25 (OH)2 vitamin D3 had no effect on the amount of matrix 45Ca binding in either strain. These results suggest that 1,25 (OH)2 vitamin D3 induces an increase in intracellular protein synthesis that results in enhanced 45Ca uptake. The similar responses of the two strains indicate that hypertensive smooth muscle is not more sensitive to 1,25 (OH)2 vitamin D3 and the Ca2+ response is a general property of vascular muscle.     

Force-velocity relationships in hypertensive arterial smooth muscle

Increased total peripheral resistance is the cardinal haemodynamic disorder in essential hypertension. This could be secondary to alterations in the mechanical properties of vascular smooth muscle. Adequate study has not been made of the force-velocity (F-V) relationship in hypertensive arterial smooth muscle. Increased shortening in arterial smooth muscle would result in greater narrowing of arteries. The objectives of this investigation were to see if there is (i) increased shortening or increased maximum change in muscle length (delta Lmax where L stands for muscle length), (ii) an increased maximum velocity of shortening (Vmax) measured in l omicron per second where l omicron is the optimal muscle length for tension development, and (iii) a difference in maximum isometric tension (P omicron) developed in spontaneously hypertensive rat (SHR; N = 6) compared with normotensive Wistar Kyoto rat (WKY;N = 5) caudal artery strips. An electromagnetic muscle lever was employed in recording force-velocity data. Analysis of these data revealed the following: (a) the SHR mean P omicron of 6.21 +/- 1.01 N/cm2 was not different from the mean WKY P omicron of 6.97 +/- 1.64 N/cm2 (p greater than 0.05); (b) the SHR preparations showed greater shortening for all loads imposed; (c) the SHR Vmax of 0.016 l omicron/s was greater than the WKY Vmax of 0.013 l omicron/s (p less than 0.05). This study provides evidence that while hypertensive arterial smooth muscle is not able to produce more force than normotensive arterial smooth muscle, it is capable of faster and greater shortening. The latter could result in increased narrowing of hypertensive arteries and increased blood pressure.     

Chronic all-trans retinoic acid treatment prevents medial thickening of intramyocardial and intrarenal arteries in spontaneously hypertensive rats

There are in vitro data linking all-trans retinoic acid (atRA) with inhibition of hypertrophy and hyperplasia in cardiomyocytes, vascular smooth muscle cells, and fibroblasts. In the present study, we tested the hypothesis that chronic treatment with atRA may blunt the process of myocardial remodeling in spontaneously hypertensive rats (SHR). Four-week-old male SHR were treated with atRA (5 or 10 mg.kg-1.day-1) given daily for 3 mo by gavage; age- and sex-matched Wistar-Kyoto rats (WKY) and placebo-treated SHR served as controls. At the end of the treatment period, cardiac geometry and function were assessed by Doppler echocardiography. Histological examination and RIA were performed to evaluate medial thickening of intramyocardial and renal arteries, perivascular and interstitial collagen content, and atrial natriuretic peptide (ANP) and IGF-I in the heart, respectively. The novel finding of the present study is that atRA prevented hypertrophy of intramyocardial and intrarenal arteries and ventricular fibrosis. However, atRA treatment did not lower blood pressure or left ventricular weight and left ventricular weight-to-body weight ratio in SHR. atRA did not change cardiac geometry and function as assessed by Doppler echocardiography. atRA showed no influence on either ANP or IGF-I levels. In conclusion, the present study suggests that chronic atRA treatment prevents medial thickening of intramyocardial and intrarenal arteries and ventricular fibrosis during the development of hypertension. Left ventricular hypertrophy and cardiac geometry and function are not changed by atRA treatment.     

Differential stimulation of growth related metabolism in cultured smooth muscle cells from SHR and WKY rats by combinations of EGF and LDL

We have reported previously that vascular smooth muscle cells from spontaneously hypertensive rats (SHR) were more responsive to epidermal growth factor (EGF) than their normotensive derived Wistar Kyoto (WKY) controls. This differential responsiveness is evident for several cellular processes including activation of S6-kinase, elevation of intracellular pH and stimulation of both phosphoinositide metabolism and DNA synthesis. Quiescent smooth muscle cells exposed to low density lipoprotein (LDL) exhibited a similar differential responsiveness (SHR greater than WKY) in terms of S6-kinase activation, which was time- and dose-dependent (10(-10)-10(7) M), but neither cell type responded appreciably to LDL in terms of a stimulation in [3H]-thymidine incorporation. Exposure of the same cells to EGF and LDL in combination elicited a marked synergistic stimulation in DNA synthesis, the extent of which was greater for SHR than WKY. The sensitivity of both cell types to EGF was increased in the presence of LDL, although cells from hypertensive animals still exhibited their greater (vs. WKY) sensitivity. In both cell types, activation of nuclear protooncogenes c-fos and c-myc by LDL was minimal, whereas oncogene induction by EGF was approximately five-fold greater for SHR-derived cells compared to those from WKY animals. No marked synergistic effect on the time-dependent induction of either entity was observed for cells exposed to EGF and LDL simultaneously, and the response of SHR-cells remained greater than WKY-cells.     

Chloride channel activity of vascular smooth muscle in the spontaneous hypertensive rats

To characterize the activity of the Ca2+-activated Cl- channels in vascular smooth muscle (VSM) of the spontaneous hypertensive rats (SHR), the isolated mesenteric vascular beds and tail artery strips were preparated from SHR and Wistar rats aged 7-8 weeks. The changes in contractile response to norepinphrine (NE) were taken as an index of vascular mortion. Results showed that the contractile responses of mesenteric arteries and tail arteries to NE in SHR were significantly greater than that in Wistar rats. The inhibition magnitude of the contractile response by Ca2+-activated Cl- channel blocker, niflumic acid in SHR was significantly less than that in Wistar rats. Decreasing the extracellular Cl- concentration increased the contractile response to NE significantly, but the amplitude of enhanced contractile response in SHR was greater than that in Wistar rats. It can be concluded that NE-induced contraction was enhanced in SHR, which is partly due to an increase in Cl- efflux through the Ca2+-activated Cl- channels. The chloride channel activity may be increased in association with the elevation of blood pressure.