Spontaneously Hypertensive Rats Myocytes Research Articles

Influence of age on contractile response to insulin-like growth factor 1 in ventricular myocytes from spontaneously hypertensive rats.

Evidence suggests a pathophysiological role of insulin-like growth factor 1 (IGF-1) in hypertension. Cardiac function is altered with advanced age, similar to hypertension. Accordingly, the effects of IGF-1 on cardiac myocyte shortening and intracellular Ca(2+) were evaluated in hypertension at different ages. Ventricular myocytes were isolated from Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR), aged 12 and 36 weeks. Mechanical and intracellular Ca(2+) properties were examined by edge-detection and fluorescence microscopy. At 12 weeks, IGF-1 (1 to 500 ng/mL) increased peak twitch amplitude (PTA) and FFI changes (DeltaFFI) in a dose-dependent manner in WKY myocytes, with maximal increases of 27.5% and 35.2%, respectively. However, IGF-1 failed to exert any action on PTA and DeltaFFI in the age-matched SHR myocytes. Interestingly, at 36 weeks, IGF-1 failed to exert any response in WKY myocytes but depressed both PTA and DeltaFFI in a dose-dependent manner in SHR myocytes, with maximal inhibitions of 40.5% and 16.1%, respectively. Myocytes from SHR or 36-week WKY were less sensitive to norepinephrine (1 micromol/L) and KCl (30 mmol/L). Pretreatment with nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 micromol/L) did not alter the IGF-1-induced response in 12-week WKY myocytes but unmasked a positive action in 12-week SHR and 36-week WKY myocytes. L-NAME also significantly attenuated IGF-1-induced depression in 36-week SHR myocytes. In addition, the Ca(2+) channel opener Bay K8644 (1 micromol/L) abolished IGF-1-induced cardiac depression in 36-week SHR myocytes. Collectively, these results suggest that the IGF-1-induced cardiac contractile response was reduced with advanced age as well as with hypertension. Alterations in nitric oxide and intracellular Ca(2+) modulation may underlie, in part, the resistance to IGF-1 in hypertension and advanced age.

Cross-talk Between Cardiacκ-Opioid andβ-Adrenergic Receptors in Developing Hypertensive Rats

Theκ-opioid receptor exerts a negative modulatory action on theβ-adrenoceptor and the action is blunted in adult spontaneously hypertensive rats (SHR). In order to determine whether the blunted negative modulation of theβ-adrenoceptor by theκ-opioid receptor contributes to the development of hypertension, the electrically induced intracellular calcium ([Ca2+]i) transient was measured in single ventricular myocytes of SHR at 4, 6, 8 and 13-week-old and the age-matched Wistar Kyoto (WKY) rats. The electrically induced [Ca2+]itransients were augmented by norepinephrine (NE), aβ-adrenoceptor agonist, over four-fold in WKY rats of all ages studied and in SHR of 4 and 6 weeks of age. The enhancing effect of NE in 8- and 13-week-old SHR was, however, only approximately three-fold, significantly lower than the corresponding values in age-matched WKY rats. Similarly, the electrically induced [Ca2+]itransients were also augmented by forskolin, an activator of adenylate cyclase, by approximately two-fold in WKY rats of all ages and SHR aged 4 and 6 weeks. In SHR aged 8 and 13 weeks, the effect of forskolin was only 1.5-fold, significantly lower than the two-fold increase in the corresponding WKY rats. The enhancing effects of NE and forskolin were attenuated by U50,488H, a selectiveκ-opioid agonist, by approximately 50 and 25%, respectively, in both types of rats of all ages studied, with the exception of 13-week-old rats. In rats of this age group, the attenuations by U50,488H on the enhancing effects of NE and forskolin were 17 and 9% in SHR, respectively, significantly less than the corresponding 54 and 29% in WKY. The fact that attenuation of U50,488H on the enhancing effects of NE and forskolin only occurs in 13-week-old SHR when hypertension has been fully developed indicates that the attenuated inhibitory modulation ofκ-opioid receptor stimulation does not contribute to the initiation of hypertension. Interestingly, the enhancing effects of NE and forskolin on the electrically induced [Ca2+]itransient was attenuated in SHR aged from 8 weeks when the blood pressure was rapidly increasing. The different time courses of altered responses to U50,488H, and NE and forskolin suggest that the attenuated negative modulation ofκ-receptor stimulation on theβ-adrenergic receptor is not due to the signal transduction pathway activated byβ-adrenergic stimulation. In 13-week-old SHR with the arterial blood pressure restored to normal by pharmacological manipulations, the blunted responses to NE, U50,488H and forskolin still occurred, indicating that the altered responses to activation ofβ-adrenergic andκ-opioid receptors and adenylate cyclase are not secondary to hypertension.

Increased Contraction of Myocytes Isolated From the Young Spontaneously Hypertensive Rat: Relationship Between Systolic and Diastolic Function

This study was designed to assess heart performance in young (10-week-old) spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats, in terms of whole heart function in vivo and mechanics of isolated ventricular myocytes in vitro. The data suggest that left ventricular pressure (LVP) generation is greater, and the maximal velocities of LVP generation and decline are faster in SHR than in WKY. Two-dimensional morphologic measurements show that SHR myocytes are hypertrophied and that augmented contractile function is also present in isolated cells as determined by the extent of shortening and velocity of shortening. Relaxation is also faster at the myocyte level as determined by velocity of relengthening. However, the slope of the relationship between myocyte peak shortening and velocity of relaxation was similar in both groups. These results suggest that hyperdynamic myocyte relengthening may reflect changes in elastic recoil from increased shortening rather than intrinsic changes in cellular mechanisms, which are independent of shortening.

Augmented Contributions of Voltage-Gated Ca2 Channels to Contractile Responses in Spontaneously Hypertensive Rat Mesenteric Arteries

The observation that organic Ca 2+ channel blockers are more effective in lowering blood pressure and peripheral resistance in hypertensive compared to normotensive subjects suggests that there is a greater contribution from voltage-gated Ca 2+ channels (Ca L) to vascular force maintenance in hypertensive arteries. This study tests this hypothesis by comparing the effects of Bay k 8644 and nisoldipine on basal force development, contractile responses to norepinephrine and serotonin, and Ca 2+ currents (I Ca) in mesenteric artery (MA) from Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). MA rings were used to record isometric contractions at L max. Single cells were isolated by collagenase plus elastase for measurement of Ca L properties by patch-clamp methods. Contractile responses to Bay k 8644 were larger and more sensitive in SHR than WKY, and were larger in endothelium-denuded compared to intact rings. In SHR, the addition of 10 nmol/L Bay k 8644 increased contractile sensitivity to norepinephrine (NE) and serotonin (5HT), and increased maximum response to 5HT. In WKY, 10 nmol/L Bay k 8644 produced a small increase in 5HT sensitivity with no effect on maximum response, and had no effect on NE responses. In the presence of 1 μmol/L nisoldipine, the maximum response and the sensitivity to both NE and 5HT were decreased in both WKY and SHR with the inhibitory effects of nisoldipine being larger in SHR than WKY. Peak I Ca was larger in SHR, and current-voltage curves were shifted toward more negative voltages compared to WKY. Bay k 8644 increased I Ca in both WKY and SHR myocytes with no apparent difference in the magnitude of its effect when expressed as a percent of control I Ca. These results suggest that Ca L contribute significantly to tonic force maintenance as well as to agonist responses in MA from both WKY and SHR, but with a much larger contribution in SHR. Differences in the sensitivity of Ca L to Bay k 8644 were not responsible for the differences in contractile responses to this agonist.

Effects of Age on Ca 2+ 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.

Ionic basis of action potential prolongation of hypertrophied cardiac myocytes isolated from hypertensive rats of different ages

The aim was to determine the ionic basis of action potential lengthening associated with cardiac hypertrophy. Action potentials and ionic currents of left ventricular myocytes isolated from normal and hypertrophied hearts were recorded with patch pipettes. Since cardiac hypertrophy is a time dependent process, myocytes isolated from hearts of spontaneously hypertensive rats (SHR) of different ages (3 and 18 months old) were compared with those of age matched normotensive controls (Wistar Kyoto rats, WKY). Membrane capacitance, an index of cell size, was significantly higher in SHR than WKY. The degree of action potential prolongation was independently correlated with hypertension and its duration, resulting in a statistically significant lengthening of action potential in 18 month old SHR compared to age matched WKY and to 3 month old SHR. ICa,L density was not significantly modified in hypertrophied myocytes compared to normal controls. Depolarising steps positive to -40 mV activated an outward current, which was composed of both transient (ItO) and maintained components (Ilate) and played a major role in controlling repolarisation in rat ventricular myocytes. Ito density was significantly reduced in SHR myocytes compared to age matched WKY and was also smaller in 18 month old compared to 3 month old SHR. Ilate was not statistically different in 3 and 18 month old SHR and WKY. Current-voltage relationships for IK1 were completely superimposable in all groups. Prolongation of action potential in the hypertrophied heart of SHR is due to specific alterations in the repolarising potassium current Ito. The phenomenon is directly related to the duration of hypertension.

DIDS-sensitive pHi regulation in single rat cardiac myocytes in nominally HCO3-free conditions.

The fluorescent dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) was used to measure pHi in the spontaneously hypertensive rat (SHR) and in normal rat cardiac myocytes under nominally HCO3-free (20 mmol/L HEPES-buffered) conditions. When only the Na-H exchanger was blocked, the intrinsic buffering power (beta i) in SHR myocytes was significantly higher than when both the Na-H exchanger and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS)-sensitive pHi regulators (the Na-HCO3 cotransporter and the Cl-HCO3 exchanger) were blocked. Similar low values for beta i were also found for normal rat myocytes in Na(+)-free conditions. In Cl(-)-free solution under nominally HCO3-free conditions, in both normal and SHR myocytes, the pHi slowly alkalinized (by 0.16 +/- 0.02 and 0.11 +/- 0.02 pH units, respectively); this alkalinization was also DIDS sensitive. The reacidification during NH4+ perfusion was inhibited 30.2 +/- 7.4% by DIDS. In addition, in the nominal absence of HCO3-, 100 mumol/L ATP acidified the pHi in both normal and SHR myocytes (by 0.21 +/- 0.03 and 0.33 +/- 0.03 pH units, respectively); this acidification was totally inhibited by 0.1 mmol/L DIDS. It has been shown, in rat cardiac myocytes, that ATP acidifies the pHi by 0.35 pH unit via stimulation of a DIDS-sensitive Cl-HCO3 exchanger in HCO3-containing solutions. Finally, we have shown, in normal cardiac myocytes, that two potent Na-H exchanger blockers, N-5-ethylisopropyl amiloride (EIPA) and N-5-methyl-N-isobutyl amiloride (MIA), only partially inhibited the pHi recovery from internal acidosis under nominally bicarbonate-free conditions. When DIDS was added at the same time as EIPA, pHi recovery from an internal acid loading was completely inhibited. Our results clearly demonstrate that in both normal and SHR cardiac myocytes, bicarbonate-dependent pHi regulators can be significantly activated under resting or acidified pHi in HEPES-buffered medium, probably because of the cellular production of CO2. The contribution of these bicarbonate-dependent pHi regulators, ie, the Na-HCO3 cotransporter and the Cl-HCO3 exchanger, cannot therefore be ignored even under nominally HCO3-free conditions.

Elevated density and altered pharmacologic properties of myocardial calcium current of the spontaneously hypertensive rat

The membrane current mediated by the L-type calcium channel (ICa) was studied for myocytes isolated from the ventricle of 10- to 11-week-old spontaneously hypertensive rats (SHR). Compared with age-matched normotensive Wistar-Kyoto (WKY) or Sprague-Dawley rats, the amplitude of ICa was greater for the SHR. Two observations suggest that the greater ICa of the SHR was not due to hypertrophy. First, the similarity of membrane capacitance for these three strains of rat indicated lack of hypertrophy. Secondly, the amplitude of ICa was also greater for myocytes isolated from the right ventricle of the SHR. The ICa of the SHR was more sensitive to drugs known to activate calcium channels via phosphorylation. Specifically, extracellular application of 1 mumol/l isoprenaline as well as intracellular dialysis with either 1 mmol/l cyclic AMP or with 1 mmol/l adenosine 5'-O-3-thiotriphosphate increased the mean ICa of SHR myocytes significantly more than that of WKY rat cells. The ICa of SHR myocytes was also more sensitive to BAY K 8644 and its enantiomorphs. The present data suggest that the greater peak amplitude of ICa for SHR myocytes relative to that of myocytes of normotensive rats is due to an increase in current density and enhancement of channel phosphorylation.

Investigation of the mechanisms underlying the increased contraction of hypertrophied ventricular myocytes isolated from the spontaneously hypertensive rat

The aim was to investigate the cellular mechanisms responsible for the increased contraction of left ventricular myocytes isolated from the spontaneously hypertensive rat (SHR). Single myocytes were isolated enzymatically from the left ventricles of SHR, Wistar-Kyoto (WKY), and Wistar rats. WKY and Wistar myocytes were used as normotensive controls. Cytoplasmic calcium was measured with Fura-2 and contraction was measured optically. Membrane potential was measured with microelectrodes and cells were voltage clamped to measure the amplitude of L-type calcium current (iCa). Under action potential conditions, SHR myocytes had a larger calcium transient and an increased sarcoplasmic reticular calcium content compared to normotensive myocytes. There was no detectable difference in the resting cytoplasmic calcium concentration between SHR and control myocytes. SHR myocytes also had a prolonged action potential compared to normotensive cells. However, when cells were voltage clamped and short pulses of 120 ms duration were applied (a similar duration of depolarisation to the action potential), the difference in the calcium transient or contraction between SHR and normotensive myocytes was abolished. SHR myocytes had an unchanged amplitude of ICa in comparison to control myocytes, and there was no detectable difference in the myofilament response to calcium between SHR and control myocytes. (1) Hypertrophied SHR myocytes stimulated with action potentials had an increased calcium transient compared to normotensive cells. The greater calcium transient in the SHR cells is likely to be a major factor responsible for their increased contraction. (2) SHR myocytes had a prolonged action potential in comparison to normotensive cells. (3) The amplitude of ICa and myofilament response to calcium were unchanged in SHR myocytes, suggesting that these factors do not play a role in the increased contraction of these cells. (4) Since the difference between SHR and control cells was abolished by voltage clamping the cells to prevent the difference of action potential, it is unlikely that an alteration of intrinsic mechanisms in SHR myocytes is responsible for their increased contraction. Rather, it suggests that the prolonged action potential of SHR myocytes plays a important role in causing their increased calcium transient and contraction. Our results indicate that the prolonged action potential in SHR cells results in an increased calcium content of the sarcoplasmic reticulum, which leads to a greater sarcoplasmic reticular calcium release upon stimulation and an increased contraction.

The electrophysiological characteristics of hypertrophied ventricular myocytes from the spontaneously hypertensive rat

Previous studies on multicellular preparations have shown that hypertrophied cardiac muscle from the spontaneously hypertensive rat (SHR) has a prolonged action potential. The first aim of the present study was to determine whether the action potential of isolated left ventricular myocytes was similarly prolonged and to study the underlying membrane currents that might be responsible. The second aim was to evaluate the L-type calcium current amplitude of SHR myocytes, as we have recently shown that they have an increased contraction and an increase in the calcium trigger entering via the L-type calcium channel might be one possible mechanism for this. The electrophysiological characteristics of left ventricular myocytes isolated from the SHR were compared with those from normotensive control rats. Action potentials were recorded with microelectrodes. Cells were voltage-clamped and the membrane currents elicited by steps to different potentials were analysed. Blockers of potassium and calcium currents were used to reveal the contribution made by these currents to net membrane currents. SHR myocytes had prolonged action potentials. The action potential duration of SHR myocytes at 90% repolarization was found to be longer, although at 20% and 50% repolarization no difference was found. There was no difference in the resting membrane potential between SHR and control myocytes. Using a voltage clamp we studied the L-type calcium current and potassium currents. The major change in SHR myocytes was a decrease in the magnitude (normalized to the membrane capacitance) of the inward rectifier potassium current elicited by negative potentials. There was no detectable difference in either the transient outward or delayed rectifier potassium currents. We also found no difference in the magnitude, time course or voltage dependence of L-type calcium current in hypertrophied SHR myocytes. First, the action potential of SHR myocytes was prolonged compared with control myocytes. Secondly, the main change in SHR myocytes was that pulses to negative potentials elicited a lower inward rectifier potassium current. A reduction in the density of inward rectifier channels might play a role in prolonging the SHR action potential, since a lower outward repolarizing current will flow through inward rectifier potassium channels during the SHR action potential repolarization. Thirdly, there was no difference in L-type calcium current density or time course between SHR and control myocytes. Thus, a change in L-type calcium current probably plays no role in causing the prolonged SHR action potential or the increased contraction of hypertrophied SHR ventricular myocytes. Finally, the prolonged action potential in SHR myocytes may itself be one factor responsible for the increased contraction of these cells.

Differential growth of neonatal WKY and SHR ventricular myocytes within sympathetic co-cultures

Sympathetic innervation is known to increase heart size in the immature animal, yet the mechanism for this growth remains to be established. This comparative study stereologically quantified the volume of cultured neonatal ventricular myocytes with and without in vitro sympathetic innervation to isolate the mechanisms regulating cardiac growth. Since ventricular myocyte size at birth differs between the spontaneously hypertensive rat (SHR) and the normotensive Wistar-Kyoto (WKY), we questioned whether SHR myocytes respond differently than WKY myocytes to innervation. Four groups of ventricular myocytes from each strain were compared: myocytes grown alone, myocytes innervated by cultured sympathetic neurons, innervated myocytes exposed to adrenoceptor blockade, and non-innervated myocytes in co-culture dishes. Volumes for the myocyte, nucleus, cytoplasm, mitochondria, sarcomeres and other cellular organelles were assessed within each population and between populations. Relative volumes were determined for the mitochondria, sarcomeres, and other cellular components within the cytoplasm. Innervated WKY myocytes were 38% larger than control myocytes ( P<0.0004). This growth was not blocked by adrenoceptor blockade ( P=0.89 vs. innervated) and was present in the non-innervated myocytes distant from the neurons in the co-cultures ( P=0.39 vs. innervated). SHR myocytes were 36% larger than WKY myocytes ( P<0.009) but did not increase with innervation ( P=0.48). SHR myocyte size was also unaffected by adrenoceptor blockade ( P=0.39) or presence of the neurons in the culture dish ( P=0.53). Neonatal WKY ventricular myocyte growth can be provoked in vitro by sympathetic innervation via regulatory mechanisms independent of neuroeffector transmission or anatomic contact, whereas volume of neonatal SHR myocytes is unaltered by sympathetic co-culture. These findings are significant for understanding normal as well as aberrant cardiomyocyte growth.

Cytosolic pH in cultured cardiac myocytes and fibroblasts from newborn spontaneously hypertensive rats.

Newborn spontaneously hypertensive rats (SHR) develop cardiac hypertrophy before a rise in blood pressure. Cytosolic pH (pHi) has been discovered to modulate cell growth and proliferation; therefore, we have investigated pHi in myocytes and fibroblasts from 3- to 4-day-old SHR and normotensive Wistar (W) and Wistar-Kyoto controls (WKY). The ratio of heart to body weight was higher in SHR than in W and WKY (7.56 +/- 0.10 v 6.21 +/- 0.10 and 5.98 +/- 0.14 mg/g in 10, 5, and 7 groups of 20 to 40 animals; P less than .001 for both). Cytosolic pH, determined with the fluorescent probe BCECF, was measured from the sixth to the eighth day in culture on confluent cells. The mean pHi was higher in myocytes from SHR than in those from W or WKY rats (7.19 +/- 0.03, N = 30, v 7.09 +/- 0.03 and 7.11 +/- 0.02, N = 25 and 30; P = .008 and .024, respectively). In contrast, pHi was similar in fibroblasts from the three strains (7.21 +/- 0.03, 7.18 +/- 0.03, and 7.19 +/- 0.02, N = 15, 15, and 14, in SHR, W, and WKY rats, respectively). External acidification induced similar decreases in pHi from SHR and WKY myocytes, maintaining higher pHi values in SHR myocytes along the entire external pH (pHo) range studied. The inhibition of Na+/H+ exchange by the amiloride derivative, ethylisopropylamiloride, decreased the steady-state pHi of myocytes independently of the initial pHi values. This study demonstrated a cytosolic alkalinization in contractile cardiac cells from SHR before a significant rise in blood pressure and in the absence of hemodynamic influences and specific plasma factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of the enhanced epidermal growth factor-induced growth response of genetically hypertensive vascular myocytes.

Although enhanced growth of the vascular myocyte is believed to play a role in hypertensive cardiovascular disease, the cellular basis of altered growth regulation is not completely understood. The present study demonstrates that in the presence of 10% fetal calf serum, the logarithmic growth rate of cultured mesenteric artery myocytes of the spontaneously hypertensive rat (SHR) is similar to that of the normotensive Wistar-Kyoto (WKY) control rat. However, in the presence of low levels of fetal calf serum, SHR myocytes respond to epidermal growth factor (EGF) with increased growth, whereas WKY cells do not. This difference does not result from different numbers or affinities of EGF receptors in these cell lines. Examination of EGF-induced growth responses of SHR and WKY myocytes in the presence of varying levels of insulin or fetal calf serum indicates that, compared with WKY myocytes, SHR myocytes have a lower requirement for factors that confer competence to respond to EGF. Another property of the SHR myocytes is an elevation of free intracellular Ca2+. To determine whether a difference in cellular Ca2+ metabolism might play a role in the differential growth response, growth of myocytes in medium containing 0.25, 0.75, or 1.25 mM extracellular Ca2+ and 5% fetal calf serum was examined. Myocytes of SHR showed enhanced growth in the presence of 5% fetal calf serum at all levels of extracellular Ca2+. It is concluded that, although vascular myocytes of SHR and WKY rats have the capacity to grow at similar rates, under limiting conditions, the SHR myocyte growth response is enhanced.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular Ca2 + metabolism of isolated resistance arteries and cultured vascular myocytes of spontaneously hypertensive and Wistar-Kyoto normotensive rats

Although alterations in Ca2+ metabolism have been demonstrated in subcultured aortic myocytes of spontaneously hypertensive rats (SHR), changes in intact tissue have not been described. This study compares Ca2+ metabolism in intact mesenteric resistance arteries and in myocytes that were derived from mesenteric arteries and maintained in primary and long-term culture. Using fura-2, basal levels of Ca2+ were found to be similar in intact vessels of SHR and Wistar-Kyoto normotensive rats (WKY), and in primary and first-passage myocytes of the two strains. During subculture, basal levels of Ca2+ became elevated in myocytes of SHR. When norepinephrine-induced Ca2+ mobilization was examined, the threshold of resistance arteries was lower in SHR, but differences were not detected with higher concentrations of the agonist. Norepinephrine-induced Ca2+ mobilization also did not differ between primary myocytes of the two strains. Angiotensin II elicited greater intracellular Ca2+ responses in myocytes of SHR at passages 1, 3 and 5. Cell growth was assessed at each passage level. While no strain differences were detected in primary, first- and second-passage cells, the growth rate became elevated in SHR in subsequent passages. These results are consistent with the hypothesis that vascular myocytes cultured from SHR with established hypertension exhibit differences in Ca2+ metabolism that are not present in the intact vessel wall. Furthermore, intracellular Ca2+ appears to be elevated in myocytes of SHR when the rate of proliferation is increased.