Effects of Bay K 8644 in aorta from spontaneously hypertensive and Wistar Kyoto rats of different ages

Aging Reduces L-Type Calcium Channel Current and the Vasodilatory Response of Small Mesenteric Arteries to Calcium Channel Blockers

Calcium channel blockers (CCBs) are widely used to treat cardiovascular disease (CVD) including hypertension. As aging is an independent risk factor for CVD, the use of CCBs increases with increasing age. Hence, this study was designed to evaluate the effect of aging on the sensitivity of small mesenteric arteries to L-type voltage-gated calcium channel (LTCC) blockers and also to investigate whether there was a concomitant change in calcium current density. Third order mesenteric arteries from male F344 rats, aged 2.5-3 months (young) and 22-26 months (old) were mounted on wire myograph to measure the tension during isometric contraction. Arteries were contracted with 100 mM KCl and were then relaxed in a cumulative concentration-response dependent manner with nifedipine (0.1 nM-1 μM), verapamil (0.1 nM-10 μM), or diltiazem (0.1 nM-10 μM). Relaxation-concentration response curves produced by cumulative concentrations of three different CCBs in arteries of old rats were shifted to the right with statistically significant IC50s. pIC50 ± s.e.m: (8.37 ± 0.06 vs. 8.04 ± 0.05, 7.40 ± 0.07 vs. 6.81 ± 0.04, and 6.58 ± 0.07 vs. 6.34 ± 0.06) in young vs. old. It was observed that the maximal contractions induced by phenylephrine and reversed by sodium nitroprusside were not different between young and old groups. However, Bay K 8644 (1 μM) increased resting tension by 23 ± 4.8% in young arteries and 4.7 ± 1.6% in old arteries. LTCC current density were also significantly lower in old arteries (-2.77 ± 0.45 pA/pF) compared to young arteries (-4.5 ± 0.40 pA/pF); with similar steady-state activation and inactivation curves. Parallel to this reduction, the expression of Cav1.2 protein was reduced by 57 ± 5% in arteries from old rats compared to those from young rats. In conclusion, our results suggest that aging reduces the response of small mesenteric arteries to the vasodilatory effect of the CCBs and this may be due to, at least in part, reduced current density of LTCC.

Attenuation of L-type Ca²⁺ channel expression and vasomotor response in the aorta with age in both Wistar-Kyoto and spontaneously hypertensive rats

Age-related vascular diseases are induced by vascular dysfunction, which involves changes in the vasomotor response. The voltage-dependent L-type calcium channel (VDCC) protein is involved in the regulation of vessel function (contraction/relaxation action). In the present study, we evaluated age-related vasomotor function and expression of the signal-related target proteins, including VDCC, using thoracic aorta from both 8- and 40-week old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). In contraction experiments using aortic rings, vasomotor responses of both phenylephrine-induced contraction and acetylcholine-induced relaxation were significantly attenuated with age in SHR, whereas WKY did not lose activity with age. Contraction induced by angiotensin II was impaired only for the 40-week old SHR among all the rat groups tested, although enhanced AT1R/reduced AT2R expression with age was observed for both WKY and SHR. In contrast, a vasomotor responsiveness to Bay K 8644 (a VDCC agonist) at the initial contraction phase was significantly attenuated in both 40-week WKY and SHR with significant reduction of VDCC protein expression. The reduced VDCC expression in 40-week old rats significantly lowered the relaxation activity of VDCC blockers, such as verapamil and Trp-His, but did not affect that of nifedipine. Taken together, we provided the first evidence that aging caused a reduction of VDCC expression in rat aorta, irrespective of the rat strain, along with diminishment of the therapeutic potential of VDCC blockers.

Age-related changes in vascular adrenergic signaling: clinical and mechanistic implications

A large and growing segment of the general population are age 65 or older, and this percentage will continue to rise. Primary care of this population has, and is becoming a priority for clinicians. Hypertension, orthostatic hypotension, arterial insufficiency, and atherosclerosis are common disorders in the elderly that lead to significant morbidity and mortality. One common factor to these conditions is an age-related decline in beta-adrenergic receptor (beta-AR)-mediated function and subsequent cAMP generation. Presently, there is no single cellular factor that can explain this age-related decline, and thus the primary cause of this homeostatic imbalance is yet to be identified. However, the etiology is clearly associated with an age-related change in the ability of beta-AR receptor to respond to agonist at the cellular level. This article will review what is presently understood regarding the molecular and biochemical basis of age-impaired beta-AR receptor-mediated signaling. A fundamental understanding of why beta-AR-mediated vasorelaxation is impaired with age will provide new insights and innovative strategies for the management of the multiple clinical disorders that effect older people.

Ca(2+) influx is increased in 2-kidney, 1-clip hypertensive rat aorta

Arteries from hypertensive rats show a greater contraction in response to Ca(2+) channel activator and an increased sensitivity to Ca(2+) entry blockers compared with those of normotensive rats. These facts suggest an altered Ca(2+) influx through membrane channels. In this study, this hypothesis was tested by direct activation of voltage-gated Ca(2+) channels using Bay K 8644, a dihydropyridine sensitive large conductance (L-type) Ca(2+) channel opener in aortas from 2-kidney, 1-clip (2K1C) hypertensive rats. Because the membrane potential of smooth muscle cells is an important regulator of the conformational state of L-type Ca(2+) channels and, consequently, dihydropyridine affinity, the effect of 10 mmol/L KCl on the responses to Bay K 8644 was also studied. Maximal contraction (ME) and sensitivity to Bay K 8644 were greater in 2K1C rats than in 2K normotensive rats (ME, 1.77+/-0.15 versus 1.25+/-0.19 g; negative log molar value [pD(2)], 8.27+/-0.07 versus 7.92+/-0.08). When the KCl concentration was increased from 4.7 to 10 mmol/L in the bathing medium, no differences were observed in the contractile effect of Bay K 8644 between 2K1C and 2K (ME, 1.28+/-0.13 versus 1.14+/-0.21 g; pD(2), 8.56+/-0.08 versus 8.38+/-0.07). The cell resting membrane potential of 2K1C aorta vascular smooth muscle cells were less negative than in 2K (-35.19+/-4.91 versus -48.32+/-1.88 mV). Basal intracellular Ca(2+) concentration ([Ca(2+)](i)) was greater in cultured vascular smooth muscle cells from 2K1C than from 2K (293.4+/-25.83 versus 205.40+/-12.83 nmol/L). In 2K1C, Bay K 8644 induced a larger increase in [Ca(2+)](i) than in 2K (190.60+/-45.65 versus 92.57+/-14.67 nmol/L), and in 10 mmol/L KCl, this difference was abolished (134.90+/-45.12 versus 125.20+/-32.17 nmol/L). The main conclusion of the present work is that the increased contractile response to Bay K 8644 in 2K1C aortas is due to an increased Ca(2+) influx through voltage-gated Ca(2+) channels.

Age-related changes in vascular responses

Aging causes changes in the structure and function of the vessels that leads to an increase in the incidence of certain cardiovascular diseases, such as hypertension, coronary artery disease, heart failure, and postural hypotension with enhancement of both morbidity and mortality. When aging is associated with hypertension, these changes are increased. Aging alters endothelial cells, and so the vascular tone regulation, reducing the endothelium-dependent relaxations, probably by a decrease in endothelial synthesis or release of nitric oxide. In addition, endothelium-independent relaxations are essentially unaltered, those elicited by beta-adrenoceptor agonists being usually reduced. Aging scarcely modifies the contractions induced by different agents, such as 5-hydroxytryptamine, histamine, high potassium, and angiotensin, whereas reduces those elicited by noradrenaline or endothelin. Vascular Ca(2+) homeostasis appears to be altered in aging. The extracellular Ca(2+) dependence of contractile responses elicited by agonists is enhanced, which explains the increased sensitivity to Ca(2+) antagonists in elderly. Finally, Na(+) pump activity, that controls cellular ionic homeostasis, seems to be reduced in aging. The contractions elicited by Na(+) pump inhibition with ouabain are negatively modulated by the release of a diffusible endothelial factor, an effect lost in aging, being replaced by an endothelium-dependent contracting factor that facilitates ouabain responses.

Effects of L-type calcium channel activation on renal vascular resistances in the Lyon hypertensive rat

This study aimed to evaluate the role of L-type calcium channels in the increased renal vascular resistance (RVR) of the Lyon hypertensive (LH) rat before and after normalization of its blood pressure (BP) by angiotensin-converting enzyme inhibition with perindopril. Concentration-response curves to Bay K 8644 (from 0.1 nM to 1 microM), a selective agonist of L-type calcium channels, were obtained in single-pass perfused kidneys isolated from groups of eight untreated or perindopril-treated (3 mg/kg/day, p.o., from age 3 to 7 weeks) LH and low-BP (LL) control rats. In untreated rats, the negative logarithm of the molar concentration of Bay K 8644 required to produce a half-maximal effect (pD2) values for Bay K 8644 did not differ between the two strains, whereas the maximal RVR response was higher in LH than in LL kidneys (28.0+/-4.9 vs. 12.9+/-0.8 mm Hg/ml/min/g, respectively). Perindopril normalized BP and RVR in LH rats and suppressed the interstrain differences in the maximal RVR responses (11.4+/-0.6 vs. 10.5+/-1.2 mm Hg/ml/min/g in LH and LL rats, respectively). These results demonstrate that LH rats exhibit an increased maximal contractile response to Bay K 8644 compared with LL controls, and that this alteration is not a primary defect because it disappears after normalization of BP level.

Effects of age on Ca2+ currents in small mesenteric artery myocytes from Wistar-Kyoto and spontaneously hypertensive rats

The purpose of this study was to test the hypothesis that differences in voltage-gated Ca2+ channels increase with age during the development of sustained hypertension in the spontaneously hypertensive rat (SHR). Using patch-clamp methods, we measured whole-cell Ca2+ currents in freshly isolated myocytes from small mesenteric arteries of juvenile (5 to 7 weeks), young (10 to 12 weeks), and mature (19 to 23 weeks) Wistar-Kyoto rats (WKY) and SHR. Indirect tail artery systolic pressure increased progressively with age in SHR (from 125 +/- 5 to 159 +/- 5 to 192 +/- 5 mm Hg) but only in the younger WKY (from 107 +/- 6 to 130 +/- 4 to 136 +/- 4 mm Hg). Peak Ca2+ current density (current per cell capacitance) was larger in SHR than WKY myocytes at all ages (at 6 weeks, 3.5 +/- 0.4 versus 2.3 +/- 0.2 pA/pF; at 12 weeks, 3.8 +/- 0.2 versus 3.1 +/- 0.2; at 20 weeks. 4.9 +/- 0.4 versus 3.3 +/- 0.4). Cell capacitance (surface area) was significantly smaller in 12-week-old SHR than WKY (25.2 +/- 1.1 versus 31.8 +/- 1.6 pF), but no differences were found in the 6- or 20-week-old groups. There were significant differences in Ca2+ current with strain, age, and voltage but no significant age-strain interactions. The ratio of peak Ca2+ current for SHR to that of WKY declined linearly with voltage at all ages suggesting differences in the voltage dependence of Ca2+ current activation. The voltage dependence of Ca2+ current was shifted to the left in SHR compared with WKY at all ages. Activation curves were shifted significantly in the negative-voltage direction only in 20-week-old SHR myocytes. We have found differences with age in Ca2+ current density and its voltage dependence in SHR compared with WKY during the phase of development in which blood pressure becomes established in the SHR. The net effect of these differences predicts a larger Ca2+ current in SHR at voltages in the physiological range of membrane potential.