Effect of treatment with cholecalciferol on the membrane potential and contractility of aortae from spontaneously hypertensive rats.

1. The diet of spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NWR) was supplemented with 12.5 micrograms cholecalciferol per 100 g body weight daily, by gavage, for 4 weeks. 2. The amplitude of the contractile responses of aortic rings from SHR to potassium and adrenaline, which was smaller than in NWR aortae, was increased after treatment with cholecalciferol. No further changes were observed in the responses of NWR and SHR aortae in the presence of 100 nM apamin. 3. The membrane potentials of aortae from SHR, which were higher than those of aortae from NWR, decreased after treatment with cholecalciferol. Further depolarization was observed in aortic rings from NWR, but not in aortic rings from SHR, after their preincubation with 100 nM apamin. 4. It is concluded that cholecalciferol normalizes the membrane potential and contractility of aortae from SHR, probably through an effect on lipid composition and structure of the plasma membrane.     

Recovery of impaired K+ channels in mesenteric arteries from spontaneously hypertensive rats by prolonged treatment with cholecalciferol.

1. The mechanism responsible for blood pressure reduction in spontaneously hypertensive rats (SHR) after prolonged cholecalciferol treatment was studied. Two-week treatment of SHR with 0.125 mg cholecalciferol kg-1 body weight per day orally caused significant reductions of systolic blood pressure and of the resting perfusion pressure of the mesenteric vascular bed at constant flow. 2. In addition, the treated animals presented a normalization of the maximum vasoconstriction response to noradrenaline and a reduction of the maximum effect of the adrenaline concentration-response curves. This latter effect probably was due to recovery of the impaired Ca(2+)-dependent K+ channels coupled to alpha 2-adrenoceptors since it was prevented by apamin. 3. The treatment with cholecalciferol also normalized the smooth muscle cell membrane potential of de-endothelialized mesenteric arteries of SHR and their hyperpolarizing responses to alpha 2-adrenergic agonists, which were depressed in untreated SHR. 4. In mesenteric rings with endothelium, alpha 2-adrenergic agonists caused similar hyperpolarizing responses in the SHR and in normotensive Wistar (NWR) and Wistar Kyoto (WKY). In non cholecalciferol-treated SHR the hyperpolarizing mediator involved in this effect was NO, while in NWR it was the endothelium-derived hyperpolarizing factor (EDHF). After cholecalciferol treatment, the hyperpolarization induced by alpha 2-adrenergic agonists in SHR smooth muscle cells was mediated by EDHF, as in NWR. 5. Our results indicate that the hypotensive effect of cholecalciferol in the SHR is probably due to the normalization of vascular reactivity, by restoring the functioning of apamin- and ATP-sensitive K+ channels located in the vascular smooth muscle cell membrane, which are impaired in the SHR.     

Different mechanism of LPS-induced vasodilation in resistance and conductance arteries from SHR and normotensive rats

1. The direct and endothelium-dependent effects of lipopolysaccharide (LPS) were investigated on resistance and conductance arteries from normotensive Wistar (NWR) and spontaneously hypertensive (SHR) rats. 2. In both NWR and SHR, LPS induced dose-dependent relaxations of the mesenteric vascular bed, which were inhibited by L-NNA in SHR but not in NWR. Iberiotoxin (IBTX) inhibited the responses to LPS in both groups, indicating the participation of high conductance Ca(2+)-dependent K(+) channels. 3. In mesenteric artery rings, the resting membrane potentials and the hyperpolarizing responses of NWR to LPS did not differ in endothelized and denuded preparations but L-NNA inhibited the responses only in endothelized rings. These responses were reduced by bosentan, suggesting that endothelin release may mask a possible hyperpolarizing response to LPS. The hyperpolarizing responses to LPS were blocked by IBTX in both endothelized and de-endothelized NWR rings. In the SHR only intact rings showed hyperpolarization to LPS, which was inhibited by IBTX and byL-NNA. 4. In SHR aortic endothelized or denuded rings, LPS induced hyperpolarizing responses which, in endothelized rings, were partially blocked by L-NNA, by IBTX or by glibenclamide, but totally abolished by IBTX plus glibenclamide. No response to LPS was observed in NWR aortic rings. 5. Our results indicate that LPS activates large conductance Ca(2+)-sensitive K(+) channels located in the smooth muscle cell membrane both directly and indirectly, through NO release from the endothelium in NWR, whereas NO is the major mediator of the LPS responses in SHR resistance vessels.     

Differences in the mechanisms for relaxation of aorta induced by 17beta-estradiol or progesterone between normotensive and hypertensive rats

The tension in isolated ring preparations of the thoracic aorta from Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) was measured isometrically to study if there are any differences in the mechanisms of 17beta-estradiol- or progesterone-induced relaxation between WKY and SHR aortic rings. 17beta-Estradiol and progesterone caused dose-dependent vascular relaxation of the thoracic aorta precontracted with norepinephrine in both WKY and SHR, and the relaxation induced by 17beta-estradiol was greater in SHR than WKY. However, no difference was observed in progesterone-induced relaxation between SHR and WKY. With the exception of tetraethylammonium, an inhibitor of Ca(2+)-activated K(+) channels, glibenclamide, a selective inhibitor of ATP-sensitive K(+) channels, or 4-aminopyridine, a selective inhibitor of voltage-dependent K(+) channels, significantly reduced 17beta-estradiol-induced relaxation only in SHR, but not in WKY. Both 17beta-estradiol and progesterone inhibited Ca(2+)-induced vasocontraction of the thoracic aorta in K(+) depolarization medium in WKY and SHR. These results suggest that the mechanisms of 17beta-estradiol-induced relaxation in SHR aorta are at least partially mediated via ATP-sensitive and voltage-sensitive K(+) channels in addition to the inhibition of Ca(2+) channels, although those of progesterone-induced relaxation in both WKY and SHR are mainly concerned with the inhibition of Ca(2+) channels rather than the operation of K(+) channels. Moreover, a difference in 17beta-estradiol-induced relaxation between WKY and SHR aorta suggests a possibility that vascular response in SHR is modified by hypertension.     

Effect of depolarizing agents on the Ca(2+)-independent and Ca(2+)-dependent release of [3H]GABA from sheep brain synaptosomes.

The purpose of the present study was to compare the effects of several depolarizing agents on both the membrane potential and on the release of [3H] gamma-aminobutyric acid (GABA) from sheep brain cortex synaptosomes. We examined the effects of KCl, 4-aminopyridine (4-AP), veratridine, ouabain and tetraphenylphosphonium cation (TPP+) on Ca(2+)-independent (carrier-mediated) and Ca(2+)-dependent (exocytotic) release. We found that, in the absence of Ca2+, KCl at 40 mM releases 7.57 +/- 0.65%, veratridine at 50 microM releases 45.85 +/- 2.48%, ouabain at 1 mM releases 8.62 +/- 0.93% and TPP+ at 1 mM releases 4.09 +/- 0.37% of the total accumulated neurotransmitter, provided that the external medium contains Na+. These are about the maximal values of release obtained with each depolarizing agent in a Na+ medium and in the absence of Ca2+. Replacing external Na+ with choline blocks the release observed in the presence of the depolarizing agents in the absence of Ca2+, and this divalent ion can increase [3H]GABA release only for K+ or 4-AP. Synaptosomal depolarization requires Na+ except for K+ depolarization. Furthermore, although Ca2+ stimulates the release of [3H]GABA due to K+ depolarization (13.56 +/- 0.44%) or due to 4-AP (4.26 +/- 0.51%), it inhibits the release due to the other depolarizing agents. The amount of [3H]GABA released by 4-AP in Na+ medium (4.26 +/- 0.51%) is similar to that induced by KCl in the presence of Ca2+ in the absence of Na+ (3.39 +/- 0.29%) which represents only exocytotic release. This suggests that the Ca(2+)-dependent exocytotic release of [3H]GABA can be specifically induced by 4-AP in a Na+ medium, or by KCl in the absence of Na+, as reported by us earlier. The observation that Ca2+ inhibits the Ca(2+)-independent release is of interest because it suggests that Ca2+ may modulate the release of cytoplasmic GABA probably by inhibiting the carrier-mediated release of GABA. It is of interest as to whether Ca2+ regulation depends on intracellular Ca2+.     

Caffeine-induced contracture in oesophageal striated muscle of normotensive and hypertensive rats

To elucidate whether properties of the sarcoplasmic reticulum are altered, not only in vascular smooth muscle, but also in visceral striated muscle of spontaneously hypertensive rats (SHR), caffeine-induced contractures in oesophageal striated muscle of Wistar Kyoto rats (WKY) and stroke-prone SHR (SHRSP) were compared. In both preparations, 30 mM caffeine induced a contracture with two components. The second component, which was diminished by extracellular Ca(2+) removal or Ni(2+) but not by verapamil, was much smaller in SHRSP. Both components and differences between WKY and SHRSP coincided with changes in intracellular Ca(2+). Although membrane potential was identical between these preparations, caffeine induced slight depolarization only in WKY preparations. Similar depolarization was observed with 10 mM K(+), which induced no contraction. It is suggested that the first and the second components of caffeine-induced contracture were induced by Ca(2+) released from sarcoplasmic reticulum and by Ca(2+) that entered through channels activated by sarcoplasmic reticulum Ca(2+) depletion, respectively. In SHRSP preparations, Ca(2+) from the latter pathway was clearly decreased, although this change is thought not to be related to the initiation of hypertension. These results suggest that Ca(2+) handling properties of cell membrane and sarcoplasmic reticulum are generally altered in muscles of SHRSP.     

Role of CA(2+)-activated K+ channels in the regulation of basilar arterial tone in spontaneously hypertensive rats

1. Ionic channels appear to play an important role in contractile responses of the cerebral arteries and, thereby, contribute to the regulation of cerebral circulation. In the present study, we investigated the role of large-conductance Ca(2+)-activated K+ (BK(Ca)) channels in the regulation of cerebral arterial tone during chronic hypertension. 2. Ring segments of the basilar artery from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were placed in bath chambers and the isometric tension of each ring was measured. 3. Application of inhibitors of BK(Ca) channels, namely tetraethylammonium (TEA; > or = 0.1 mmol/L) and charybdotoxin (CTX; > or = 0.1 nmol/L), produced spontaneous contraction with rhythmic oscillation in the basilar artery from SHR. 4. The oscillatory contraction was not induced by 5-hydroxytryptamine (0.01-10 micromol/L) or depolarization by external high K+ (20-60 mmol/L). 5. The rhythmic contraction was completely abolished by either the removal of external Ca(2+) or the application of nicardipine (10 nmol/L). 6. The oscillation was not affected by the substitution of external Cl(-) by various equimolar anions (i.e. acetate, benezenesulphonate, bromide and isethianate). 7. The amplitude of the oscillation was dose-dependently increased by the vasodilators forskolin and sodium nitroprusside, as well as by stimulation of the endothelium with histamine and acetylcholine, whereas the frequency was decreased. 8. In contrast, the oscillation was eliminated by depletion of Ca(2+) stores by caffeine. Neither TEA (10 mmol/L) nor CTX (10 nmol/L) produced any significant contraction of the basilar artery in WKY rats. 9. These results suggest that BK(Ca) channels may play an important role in regulating the resting tone of the cerebral artery in SHR.     

Alteration of arterial smooth muscle potassium channel composition and BKCa current modulation in hypertension

We investigated K+ currents and their regulation by the sarcoplasmic reticulum in mesenteric arterial smooth muscle cells of the spontaneously hypertensive rat (SHR). Using perforated patch-clamp technique, we found the overall K+ current density was significantly lower in adult SHR compared to adult Wistar-Kyoto rats (WKY). The K+ currents were almost exclusively of large-conductance Ca2+-dependent (BK(Ca)) variety in SHR, but largely of voltage-gated (Kv) variety in WKY. Western blot assay showed parallel findings. These differences were not observed in pre-hypertensive rats. Depleting the intracellular Ca2+ store inhibited the K+ currents in adult SHR. Ryanodine augmented the K+ current at 1 microM, but suppressed it at 10 microM; 2-aminoethoxydiphenyl borate demonstrated concentration-dependent inhibition. We conclude that an alteration of membrane K+ channel composition has resulted in lower overall K+ current density. The changes in K+ current type may indicate an underlying defect in Ca2+-handling that predisposes smooth muscle cells to the hypertensive phenotype.     

Effect of high extracellular Ca(2+) levels in spontaneously hypertensive rat aorta.

The release of endothelial relaxing factors has been suggested to be important in modulating the inhibition of the contractile activity caused by the increase in extracellular Ca(2+) concentration in arterial tissue. Since the hypertensive process in spontaneously hypertensive rats (SHR) could be associated with the release of endothelial vasoconstrictor factors (mainly cyclooxygenase-dependent endoperoxides and endothelin-1), we studied the contractile responses to KCl, methoxamine and phenylephrine in different aorta ring preparations (intact, de-endothelized, 10(-5) M indomethacin-treated, 10(-6) M CGS-27830 [meso-1,4-dihydro-5-methoxycarbonyl-2, 6-dimethyl-4-(3-nitrophenyl)-3-pyridine carboxylic acid anhydride]-treated, and treated simultaneously with 10(-5) M indomethacin and 10(-6) M CGS-27830) from SHR and normotensive Wistar Kyoto rats (WKY), at various Ca(2+) concentrations (1.25, 2.5, 5 and 10 mM) in the organ bath. In endothelium-intact preparations from WKY rats we observed a decrease in KCl, methoxamine and phenylephrine contractions with high Ca(2+) concentrations (5 and 10 mM), but in the endothelium-intact preparations from SHR, the increase in extracellular Ca(2+) concentration potentiated methoxamine contractions and caused no change in KCl and phenylephrine contractions. When the endothelium was disrupted in preparations from both WKY rats and SHR, we observed a decrease in KCl and methoxamine contractions with high Ca(2+) concentrations. The decrease in phenylephrine contractions caused by high Ca(2+) concentrations was clear in de-endothelized preparations from WKY rats but slight in de-endothelized preparations from SHR. In all indomethacin- and CGS-27830-treated preparations, and also in the preparations from WKY rats and SHR treated with both drugs, we observed a decrease in all the contractile responses with increased Ca(2+) concentration. Besides, there was a clear reduction in the responses of the alpha(1)-adrenoceptor agonists in the WKY and SHR preparations treated with both drugs. The results indicate that, in the hypertensive arteries, endothelium-derived contractile factors can counteract the relaxing effect of high extracellular Ca(2+) concentrations.     

Different actions of a short-acting barbiturate on sodium and potassium conductances in invertebrate and vertebrate neurons

In this study, the effects of methohexital are compared on the voltage-gated sodium (Na+) and potassium ion (K+) conductances of Retzius cells in the leech Macrobdella and of dorsal root cells of the chick in culture. Under current-clamp conditions methohexital prolonged the Na+-dependent action potential of neurons in the leech. This prolongation occurred in the absence of changes in resting membrane potential or the maximum rate of depolarization of the spike. The prolonged action potentials were identical to those recorded in the same neurons in the absence of outward currents [i.e. in Ca2+-free Ringer's solution containing Mn2+, tetraethylammonium chloride (TEA) and 4-aminopyridine (4-AP)]. They consisted of an initial spike, followed by a plateau lasting several hundreds of milliseconds. Both components of the action potential were Na+-dependent and resistant to tetrodotoxin (TTX), while the plateau was selectively blocked by saxitoxin (STX), suggesting that it originated from the flow of Na+ through a conductance different from that underlying the spike potential (Johansen and Kleinhaus, 1987). Similarly, the plateau of the action potential prolonged by methohexital, described in this study was abolished by 50 microM saxitoxin. These results suggest that the action of the drug resulted from a block of repolarizing K+-conductances. This was confirmed by voltage-clamp experiments which showed that methohexital (100-1000 microM) reduced both IK and IA in the Retzius cell, essential mimicking the combined effects of TEA and 4-AP (Johansen and Kleinhaus, 1986b). In contrast, in dorsal root cells, methohexital decreased the amplitude of Na+ and K+ currents equally. This modulation of ionic conductances by methohexital may be important for the sedative and anesthetic actions of the drug.     

Characterization of alpha2-adrenoceptors in smooth muscles of the spontaneously hypertensive rat aorta

Previous works have shown that the alpha(2)-adrenoceptor agonist UK 14,304 induced the relaxation and hyperpolarization of the rat aorta, mediated by alpha(2)-adrenoceptors present in the smooth muscles, through small-conductance, ATP-sensitive K(+) channels. We now report that in spontaneously hypertensive rat (SHR) aortic rings, UK 14,304 induced concentration-dependent hyperpolarizing responses, which were inhibited by yohimbine, an alpha(2)-adrenoceptor inhibitor, and by glibenclamide, a specific inhibitor of small-conductance, ATP-sensitive K(+) channels. The responses were also partially inhibited by iberiotoxin and by apamin. Treatment with N(omega)-nitro-L-arginine (L-NNA) did not affect the response to UK 14,304. These results indicate that alpha(2)-adrenoceptors are present in SHR aortic smooth muscle cell membranes, but differ from those of normotensive animals regarding the K(+) channels involved in their responses. Moreover, the resting membrane potential (RMP) was significantly more negative in SHR than in normotensive rats. This relative hyperpolarized state is probably due to Ca(2+)-dependent K(+) channels being constitutively open in SHR, since the addition of iberiotoxin caused a significant depolarization of the aortic smooth muscle membranes in this strain.     

The mechanism of acidic pH-induced contraction in aortae from SHR and WKY rats enhanced by increasing blood pressure.

1. Effect of pH on vascular smooth muscle contraction was analyzed by use of biochemical and pharmacological techniques. 2. In the aorta isolated from spontaneously hypertensive rats (SHR) decreasing extracellular pH (pH0) caused a rapid acidification of intracellular pH accompanied by a pH0-dependent increase in tension. The contraction of the SHR aorta was remarkable compared with that of the Wistar Kyoto rat (WKY) aorta. 3. Removal of NH4Cl caused a transient decrease in intracellular pH followed by a marked increase in tension. 4. Both contraction and intracellular Ca2+ mobilization induced by acidic pH0 were markedly inhibited by removal of extracellular Ca2+, verapamil and adenosine, whereas these were not affected by tetrodotoxin or Gd3+, a stretch-activated cation channel blocker. Furthermore, cromakalim (a K+ channel opener) inhibited acidic pH0-induced contraction (APIC). 5. Acidic pH0 induced a depolarization of cultured smooth muscle cells from SHR aorta. 6. Blood pressure elevated with increasing age of WKY and SHR accompanied by an increase in APIC. Feeding WKY with NG-nitro-L-arginine, an inhibitor of nitric oxide synthases caused a marked elevation of their blood pressure followed by an increase in APIC. 7. These results suggest that APIC is caused by Ca2+ influx mediated through the activation of voltage-sensitive Ca2+ channels mainly due to acidic pH0-induced depolarization of the plasma membrane of smooth muscle cells. It is also suggested that APIC is strengthened by the elevation of blood pressure.     

Calcium dependence of ouabain-induced contraction in aortas from spontaneously hypertensive rats

The calcium sensitivity of ouabain-induced contractions of aortic strips from spontaneously hypertensive rat (SHR) was examined using several drugs which affect Na+ and Ca2+ movements across the cell membrane, and the results were compared with those obtained with age-matched Wistar-Kyoto rat (WKY). The Ca2+ concentration-response curves (10(-3) M ouabain-treated preparations) made with aortic strips from SHR lay to the left of those made with aortic strips from WKY (Ca EC50 values: SHR, 0.51 +/- 0.16 mM, n = 6; WKY, 1.23 +/- 0.41 mM, n = 7; P less than 0.05). Amiloride (a Na+ entry blocker) and nifedipine (a Ca2+ entry blocker) attenuated the sensitivity to Ca2+ of SHR and WKY aortic strips. With 2 x 10(-4) M amiloride, WKY vessels showed a 1.3-fold increase in the Ca EC50 value and SHR a 2.1-fold increase. With 10(-6) M nifedipine. WKY vessels showed a 1.1-fold increase in the Ca EC50 value and SHR a 1.5-fold increase. Addition of monensin (Na ionophore) produced a dose-dependent potentiation in ouabain-treated aorta from WKY, but not in ouabain-treated aorta from SHR. Addition of 1.5 x 10(-5) M A23187 (Ca ionophore) eliminated the difference between the Ca2(+)-induced contractions in aortas from SHR and WKY. These results suggest that enhancement of Ca2+ influx by Na(+)-Ca2+ exchange and/or voltage-dependent Ca2+ channels in vascular smooth muscle cell membranes may be an important factor in the difference between ouabain-induced contractions in aorta from SHR and WKY.     

Vascular relaxation response to hydrogen peroxide is impaired in hypertension

1. In phenylephrine (1 microm)-precontracted rat superior mesenteric arteries (MA), hydrogen peroxide (H(2)O(2), 0.3 and 1 mm) caused a biphasic response: a transient contraction followed by a relaxation. In the presence of thromboxane A(2)/prostaglandin H(2) (TP) receptor antagonist (SQ 29548), the contractile component of the biphasic response was abolished. The relaxation response to H(2)O(2) was smaller in spontaneously hypertensive rats (SHR) when compared with normotensive Wistar-Kyoto rats (WKY). 2. The mechanisms for the attenuated relaxation to H(2)O(2) in the SHR were studied. KCl (40 mm) prevented the relaxation response. Calcium-dependent K(+) channel (K(Ca)) blockers (tetraethylammonium chloride, TEA; iberiotoxin, and charybdotoxin) showed a greater inhibition of H(2)O(2) relaxation in SHR than in WKY, whereas voltage-dependent K(+)-channel (K(v)) blocker 4-aminopyridine was more effective in inhibiting the relaxation in WKY than in SHR. 3. H(2)O(2) (1 mm) greatly enhanced the frequency and intensity of the spontaneous transient outward K(+) currents in SHR MA, and the effects of H(2)O(2) were inhibited by iberiotoxin, while in WKY MA the K(+) currents induced by H(2)O(2) were mainly of the K(v) type. The consequence of the activation of different types of K(+) channel was that the net increase in mean outward K(+) current density in response to H(2)O(2) was smaller in SHR than in WKY, which may account for the attenuated relaxation response to H(2)O(2) in the SHR. 4. The contractile responses of MA to TEA, iberiotoxin, and charybdotoxin were greater in SHR than in WKY. 5. In summary, an attenuated relaxation response to H(2)O(2) was found in SHR MA when compared to WKY. In contrast to the activation of K(v) channels in WKY, H(2)O(2) markedly enhanced K(Ca) activity in SHR, resulting in an attenuation of the increase in mean outward K(+) current density in response to H(2)O(2). These results suggest that alteration in K(+) channel activation by reactive oxygen species may play a role in the development of hypertension in SHR.     

Gene expression and functional activity of sodium/calcium exchanger enhanced in vascular smooth muscle cells of spontaneously hypertensive rats

Effects of hypertension on the function of the Na+/Ca2+ exchanger (NCX) were investigated by analyzing vascular smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats. Angiotensin II-induced 45Ca2+ efflux from VSMCs mediated by NCX was enhanced by up to 3-fold in SHR compared with WKY, whereas ionomycin-induced Ca efflux mediated by NCX was not different between SHR and WKY. The decline rate from the peak value of intracellular 45Ca2+ concentration ([Ca2+]i) mobilized by angiotensin II was decelerated by removal of extracellular sodium (Na+o) in SHR but not in WKY. Gene expressions of NCX subtype 1 and angiotensin II receptor type1A assessed by quantitative RT-PCR were increased by 1.3- and 1.5-fold, respectively in SHR compared with WKY. NCX protein was also increased 1.6-fold in SHR compared with WKY. MEK inhibitor, PD98059, partly blocked the Nao-dependent acceleration of the [Ca2+]i recovery rate and tyrosine kinase inhibitor, genistein, diminished it in SHR. Genistein decreased angiotensin II-induced Nao- dependent 45Ca2+ efflux. However, angiotensin II did not enhance the tyrosine phosphorylation of NCX. These results suggest that acceleration of Ca2+ efflux from VSMCs of SHR was at least partly due to the enhancement of functional activity of NCX via increased gene expression and tyrosine phosphorylation in connection with hypertension.     

Nifedipine, its action on the cationic concentrations in heart, vessels, skeletal muscle and blood in tissues of normotensive and spontaneously hypertensive rats (SHR)

1. The cationic tissue distribution, obtained by atomic absorption spectrophotometry, was different in normotensive (Wistar) and spontaneously hypertensive male rats (SHR). 2. In both groups, nifedipine (4.2 mg/100 g body wt, by gastric intubation, during 10 days) altered the cationic composition mainly in the aorta, atria and in SHR also in the vein. 3. In normotensive Wistar rats (NWR), nifedipine provoked a higher concentration of divalent cations (Ca2+, Mg2+ and Zn2+) in the ascending part of the aorta and reduced the monovalent (Na+) concentration in the aorta, vein and skeletal muscle. 4. In spontaneously hypertensive rats (SHR) the pathognomonic higher cationic concentrations in the aorta, right atrium and vein are significantly (P less than 0.05) reduced after nifedipine treatment.     

Quantitative changes in cardiac Na+, K+ -adenosine triphosphatase of spontaneously hypertensive rats

Sodium pumps of cardiac plasma membranes were studied in young, spontaneously hypertensive rats (SHR) and in their normotensive controls (Wistar-Kyoto; WKY) using the two following methods. The enzymatic activity and its sensitivity to ouabain were measured as the Na+, K+ -dependent ATP hydrolysis, and the number of pumps was estimated by [3H] ouabain binding. The main results of this study were the observations that (a) concentrations of ouabain as low as 10(-10) M inhibited 10-15% of the enzyme activity in both strains; (b) Na+, K+- adenosine triphosphatase (ATPase) activity in membranes from SHR was double that in membranes from WKY (16.5 +/- 3.2 mumol Pi/h/mg protein vs. 8.2 +/- 1.2 mumol Pi/h/mg protein for 10(-7) M ouabain; p less than 0.01); (c) sensitivity to three different cardiac glycosides, ouabain, digoxin, and digitoxigenin, was identical in SHR and WKY vesicles; and (d) the binding capacity of [3H] ouabain was significantly higher in SHR than in WKY vesicles, but the dissociation constant (KD) did not appear to differ between the two substrains. These studies, performed on 3-week-old rats before the appearance of hypertension, showed, on the one hand, the existence of a Na+, K+ -ATPase of very high affinity in the rat heart, and, on the other, that cardiac sarcolemmal membranes from SHR had a greater number of sodium pumps than those from WKY and thus a greater ability to extrude sodium.     

Inhibition of potassium-induced release of histamine from mast cells by tetraethylammonium and tetramethylammonium.

In accordance with our previous results, a marked release of histamine (HA) from rat peritoneal mast cells was initiated by 150 mM KCl in the absence of extracellular Ca2+. This release could be reduced by 20-60 mM tetraethylammonium (TEA) or tetramethylammonium (TMA), the non-selective K(+)-channel blockers, Ouabain, the general inhibitor of (Na+ + K+) ATP-ase, failed to produce any changes in this release. The action of TEA discriminated between the initiation of HA release evoked by different agents, producing a blockade of the K(+)-induced but not the 48/80-stimulated HA release. In total, these data suggest the presence of TEA/TMA-sensitive K(+)-channels in the mast cell membrane and their involvement in one of the possible pathways for the initiation of HA release.     

Effects of H-7 (protein kinase inhibitor) and phorbol ester on aortic strips from spontaneously hypertensive rats

We investigated the vascular responsiveness to vasoactive agents and the inhibition by H-7 (1-(5-isoquinoline-sulfonyl)-2-methylpiperazine), which inhibits cyclic nucleotide-dependent protein kinases and protein kinase C(PKC) equally potently in helically cut strips of thoracic aortas from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The susceptibility of norepinephrine (NE)- and angiotensin II (Ang II)-induced contractions to H-7 was significantly higher in the aortas from SHR than in those from WKY. H-7 decreased the contractile responses to KCl to a similar extent in both strains without affecting the high K(+)-stimulated Ca2+ influx. H-7 produced a shift to the right of the dose-response curve for the PKC activator, 12-o-tetradecanoylphorbol-13-acetate (TPA) in the case of SHR aortas, while no such shift was noted in tissues from WKY. Functional alterations in the PKC branch of the Ca2+ messenger system in vascular smooth muscle may play an important role in SHR during the sustained contraction.     

22Na+ and 86Rb+ transport in vascular smooth muscle of SHR, Wistar Kyoto, and Wistar rats

To gain further insight into differences in cellular Na+ and K+ regulation between the spontaneously hypertensive rat (SHR), Wistar Kyoto (WKY), and American Wistar (W) rats, 22Na+ and 86Rb+ washouts were performed under steady-state conditions in cultured vascular smooth muscle cells from the three rat strains. SHR vascular smooth muscle cells showed significantly higher bumetanide sensitive 86Rb+ washout rate constant (x 10(-4)/min; mean +/- SEM) than WKY cells (-38.6 +/- 2.84 and -23.8 +/- 3.58, respectively; p less than 0.005). SHR vascular smooth muscle cells also exhibited significantly higher values than WKY cells in the total 22Na+ washout rate constant (x 10(-2)/min) (-61.0 +/- 1.57 vs. -53.8 +/- 1.24; p less than 0.005). The amiloride sensitive component of the 22Na+ washout rate constant accounted for these differences (-18.6 +/- 1.04 for SHR and -12.1 +/- 2.00 for WKY; p less than 0.05). There were no apparent differences in cellular Na+ concentrations between WKY and SHR cells. In general, the 86Rb+ and 22Na+ washout parameters of W rat cells were quite similar to those of cells from SHR. We conclude that the bumetanide-sensitive 86Rb+ washout (the Na+ K+-cotransport), the overall, and the amiloride-sensitive 22Na+ washout (the latter primarily represents the Na+/H+ antiport) are higher in SHR than WKY rat vascular smooth muscle cells. These findings indicate innate differences in cellular Na+ and K+ transport in vascular smooth muscle cells of the SHR and WKY rat. The mechanisms responsible for these differences are yet to be determined.