Rhythmic contractions of isolated,pressurized small arteries from rat

The present study was undertaken to examine the influence of transmural pressure on vasomotion and to determine if any such influence was endothelium–dependent. Responses to changes in intravascular pressure of cannulated mesenteric small arteries were investigated under no–flow conditions. Both intact and endothelium–denuded arteries dilated passively when intravascular pressure was increased stepwise from 20 to 140 mmHg. When tone was induced by noradrenaline, pressure increase resulted only in dilatation, independent of endothelium. The sensitivity to noradrenaline was significantly increased in vessels without endothelium, indicating a relaxing influence of the endothelium. Rhythmic contractions in response to noradrenaline occurred in all intact arteries, but were absent when the endothelium was removed. The amplitude of the rhythmic contractions decreased significantly when transmural pressure was elevated. The frequency increased when pressure was elevated from 20 to 80 mmHg and then remained rather constant during further pressure increases. As shown previously in non–pressurized arteries, exogenous cyclic GMP induced oscillations in endothelium–denuded arteries. Pressure–related effects on vasomotion were not dependent on an intact endothelium. Ryanodine, ouabain or verapamil inhibited the rhythmic activity, confirming previous results in non–pressurized arteries. Thus, changes in transmural pressure can modulate vasomotion, but this effect does not appear to be mediated by the endothelium. Generation of vasomotion may depend on release of Ca²⁺ from intracellular stores, the activity of the Na⁺, K⁺–pump and transmembrane Ca²⁺ inflow in a pressurized artery as shown previously in these arteries under isometric conditions.     

Contribution of Na+/Ca2+ exchanger to the regulation of myogenic tone in isolated rat small arteries

The contribution of the Na⁺/Ca²⁺ exchanger to the myogenic vascular tone was examined in rat isolated skeletal muscle small arteries (A_SK) with pronounced myogenic tone and mesenteric small arteries (A_MS) with little myogenic tone. Myogenic tone was assessed by the vascular inner diameter at transmural pressures of 40 and 100 mmHg. To depress the Na⁺/Ca²⁺ exchanger, the extracellular Na⁺ concentration ([Na⁺]o) was lowered from 143 to 1.2 mM by substituting choline‐Cl for NaCl. The A_SK developed significant myogenic tone and constricted further in low [Na⁺]o. Nifedipine (1 μM ) reduced both myogenic tone and low [Na⁺]o‐induced contraction. Because the membrane potential of A_SK was not changed by low [Na⁺]o (–35 ± 2 mV at 143 mM [Na⁺]o, ?37 ± 3 mV at 1.2 mM [Na⁺]o), depolarization‐induced Ca²⁺ influx was not a cause of the low [Na⁺]o‐induced contraction. The A_MS did not develop significant myogenic tone. Although low [Na⁺]o also constricted A_MS, the magnitude of constriction was significantly weaker than that in A_SK (17 ± 4 vs. 47 ± 6%, P < 0.01, at 58 mM Na⁺). With Bay K 8644, A_MS developed myogenic tone, and low [Na⁺]o‐induced constriction was significantly increased. In conclusion, Na⁺/Ca²⁺ exchanger may play an important role in regulating myogenic tone, likely via mediating Ca²⁺‐extrusion.     

Rhythmic contractions of isolated small arteries from rat: role of calcium

In order to investigate the mechanisms behind rhythmic contractions in small arteries of the mesenteric arcade from Wistar rats, the calcium dependency of the oscillations in response to noradrenaline activation was tested on isolated vessels. Application of 1 μM ryanodine or 30 μM TMB-8 (procedures known to inhibit Ca²⁺ release from intracellular stores) totally abolished the rhythmic activity, even though the antagonists had opposite effects on the amplitude of the contractile response to noradrenaline. Verapamil (1 μ) or felodipine (1 μ M) (agents known to inhibit influx of extracellular Ca²⁺) also abolished the oscillations and reduced the maximal noradrenaline response by about 40%. Reducing the extracellular Ca²⁺ concentration to 0.1 DIM reduced the amplitude of the noradrenaline response to a similar extent as 1 nM felodipine, but did not eliminate the oscillations. This may indicate that the effect of calcium entry blockers was to eliminate the voltage-dependency of Ca²⁺ inflow rather than just reducing the Ca⁸⁺ level. Manoeuvres that would increase the cytosolic Ca²⁺ concentration (exposure to caffeine or to the calcium agonist BAY-K 8644) increased the frequency of the oscillations. These observations indicate an important role, not only for voltage-operating channels, but also for intracellular calcium stores in the generation of rhythmic contractions in these small arteries. Oscillations appear to be generated by an interplay between membrane activation and intracellular calcium stores.     

Rhythmic contractions of isolated small arteries from rat: influence of the endothelium

Small arteries of the mesenteric arcade from Wistar rats display rhythmic oscillations superimposed on the tonic contractile response when exposed to submaximal doses of noradrenaline. We have previously shown that mechanical removal of the endothelium abolishes these oscillations. In the present study different methods to eliminate or modify the influence of the endothelium were used in order to further characterize the mechanisms behind rhythmic contractions in these vessels. Endothelium was removed either mechanically or chemically by perfusing the vessels with 0.3% CHAPS. The absence of functional endothelium enhanced noradrenaline sensitivity and simultaneously abolished oscillations in tension and membrane potential, but did not affect resting membrane potential. The rhythmic activity was also reduced or abolished by exposure to haemoglobin, methylene blue, LY83583 or l -NNA. Indomethacin and propranolol were without effect. Sodium nitroprusside or the permeant analogue of cyclic GMP, 8-bromo cyclic GMP, restored rhythmic activity in precontracted endothelium-denuded vessels. The data suggest that release of nitric oxide from the endothelium, and subsequent generation of cyclic GMP in the smooth muscle, activates oscillations in membrane potential and tension; the oscillator itself appears to be located within the smooth muscle cells.     

Ionic dependence of electrical activity in small mesenteric arteries of guinea-pigs

The regulation of blood vessel diameter is under the control of the autonomic nervous system (as well as hormones and metabolites), sympathetic nerve stimulation evoking depolarizing post-synaptic potentials. Excitatory junction potentials (EJPs) were recorded from vascular smooth muscle cells of guinea-pig small mesenteric arteries (pressurized) following nerve stimulation. Repetitive stimulation (>5Hz) led to summation of EJPs, which evoked spikes and vasoconstriction. Replacing extracellular Na⁺ with choline (plus atropine) resulted in a decrease in EJP amplitude, but spike amplitude and maximum rate of rise (+V_max) were unaffected. Decreasing the extracellular Ca²⁺ concentration produced decreases in EJP amplitude and spike +V_max, while increasing extracellular Ca²⁺ resulted in increased EJP amplitude and spike +V_max. Verapamil and bepridil, agents that depress Ca²⁺ influx in vascular and visceral smooth muscle, depolarized the membrane and depressed EJPs and spikes at high concentrations (10^–5 M and 5×10^–6 M, respectively). The data indicate that EJPs are dependent on external Na⁺ and Ca²⁺ ions, and that spikes are dependent on Ca²⁺. Thus, neuromuscular transmission in this muscle is similar to that in non-vascular smooth muscles, such as intestinal muscle and vas deferens.Part of this work has been presented to the Biophysical Society (Zelcer and Sperelakis 1980) and to the American Physiological Society (Zelcer and Sperelakis 1981)     

Segmental arterial mediolysis of the middle colic artery: report of a case with special reference to lesions of small arteries and veins

We report a case of segmental arterial mediolysis (SAM) that involved the middle colic artery, and present some pathologic alterations found in mesenteric small arteries and veins. The patient, a 52-year-old woman, underwent an emergency laparotomy for acute intra-abdominal hemorrhage, and a segment of the transverse colon with hemorrhagic mesocolon was excised. Microscopic examination demonstrated two separate lesions of SAM in the middle colic artery. The involved arterial segments showed a partial to circumferential loss of the media (mediolysis) and intima associated with the formation of a pseudoaneurysm. Smooth muscle cells adjacent to the mediolysis showed various degenerative changes. In addition, we found multiple, tiny foci of degenerative lesions affecting the outer media in the wall of small arteries. Subendothelial vacuoles and nodular intimal proliferation were also noted in small veins. Whereas SAM chiefly affects large or medium-sized arteries, small blood vessels, including veins, are also affected. The present case is unusual in that degeneration of medial smooth muscle cells was clearly observed in the arterial walls, and the small veins were affected by lesions similar to those in arteries. We suspect that a phenotypic modulation of vascular smooth muscle cells induced by some genetic vulnerability plays a role in the pathogenesis of SAM.     

Electrical field stimulation of rat mesenteric small arteries: force and free cytosolic calcium during neurogenic contractions and mechanisms of non-neurogenic relaxations

The effect of transmural electrical field stimulation (TEFS) of rat mesenteric small arteries was studied. Stimulation parameters were selected to cause tetrodotoxin (TTX) sensitive contractions. In arteries precontracted with PGF_2α in the presence of phentolamine, TTX insensitive relaxation could be induced by TEFS. The relaxing effect of TEFS required higher stimulation amplitude and duration than the contractions. Thus, by appropriately choosing stimulation parameters, contractile responses could be elicited which were little affected by any relaxing effect, while contractions were abolished by TTX at any stimulation conditions in the present study. The contractions were abolished by cold storage and almost completely inhibited by phentolamine. Thus, contractions were neurogenic and primarily caused by noradrenaline. At low frequencies, TEFS caused phentolamine sensitive increases in free cytosolic calcium with no contractions. At higher frequencies, there was a further increase in free cytosolic calcium, associated with contraction. Only at high frequencies, noradrenaline from nerves caused sensitization of the contractile filaments to free cytosolic calcium as during stimulation with exogenous noradrenaline. The relaxations were associated with decreases in free cytosolic calcium and were probably non-neurogenic since they were resistant to TTX, cold storage, capsaicin, and repeated stimulation. Furthermore, relaxations were almost completely abolished by increasing extracellular potassium to 40 mM or by adding tetraethylammonium chloride or 4-aminopyridine. Relaxations were also reduced by ouabain and potassium free conditions.     

Transmitter characteristics of cutaneous,renal and skeletal muscle small arteries in the rat

Aim: We studied transmitter characteristics of proximal and distal arteries supplying skin (saphenous artery and its medial tarsal branch), kidneys (terminal branches of renal artery and interlobar arteries) and skeletal muscle (proximal and distal sections of external sural artery). Methods: Artery segments were mounted in an isometric myograph and intramural nerves were activated by electrical field stimulation. Adrenergic and purinergic components of the neurogenic response were blocked using phenoxybenzamine and α,β‐methylene adenosine triphosphate (mATP), respectively. Results: Arteries from skin or kidney developed rapid and prominent neurogenic contractile responses, with half‐maximal amplitude reached within 5–15 s; responses in proximal vessels were greater than in distal vessels. Arteries from skeletal muscle responded to sympathetic stimulation with a moderate contraction developing over 1 min or more, the response of distal segments was greater than that of proximal segments. In skeletal muscle vessels the sympathetically evoked contraction was completely blocked by phenoxybenzamine, whereas in skin and renal vessels it was the combined effect of noradrenaline and adenosine triphosphate (ATP). Given alone, mATP did not change the magnitude of the response to nerve stimulation, but increased its latency and also potentiated the response to exogenous noradrenaline. In all vascular beds, distal vessels were more sensitive to noradrenaline and mATP. Conclusion: It thus appears that the noradrenaline/ATP ratio of the sympathetic vasoconstrictor response differs between vascular beds in a way that is consistent with known differences in the selective regulation of regional vascular resistance by the sympathetic nervous system.     

The effects of thapsigargin on [Ca2+]i in isolated rat mesenteric artery vascular smooth muscle cells

Summary The effects of thapsigargin were studied on single cells isolated from side branches of the rat mesenteric artery. Thapsigargin (150 nM) produced a transient increase of [Ca²⁺]_i. This transient rise of [Ca²⁺]_i was unaffected by removing external Ca²⁺ ions. This suggests that thapsigargin is releasing Ca²⁺ ions from an intracellular store. In the absence of thapsigargin both noradrenaline and caffeine also produced a transient increase of [Ca²⁺]_i. These increases were abolished by prior exposure to thapsigargin. Correspondingly, the effects of thapsigargin were abolished by prior exposure to caffeine. These results show that thapsigargin releases Ca²⁺ from the noradrenaline and caffeine-sensitive stores.     

Mechanical properties of rat cerebral arteries as studied by a sensitive device for recording of mechanical activity in isolated small blood vessels*

A sensitive device for recording of mechanical activity in isolated small blood vessels with calibres down to 100 μm is described. This equipment was used to examine the mechanical properties of rat cerebral arteries. The ultrastructure of the preparations was investigated by light-, transmission, and scanning electron-microscopy. In general the walls of the middle cerebral and basilar arteries consisted of 3 layers of smooth muscle cells, which occupied approximately 80% of the total wall thickness. The present technique preserved the integrity of the vessel wall and caused no observable damage to the smooth muscle or endothelial cells. Neither the basilar nor the middle crebral arteries developed spontaneous phasic contractions under standard conditions. Potassium excess (124 mM) induced a biphasic contractile response characterized by a fast and partly transient increase in tension (phase A), followed by a slowly developing sustained contraction (phase B). The responses to K⁺ were strong, highly reproducible and not influenced by pH changes in the range 6.9-7.8, making K⁺-stimulation suitable for testing of vascular contractility. Length-tension measurements were performed on relaxed and K⁺-activated basilar arteries. The mechanical behaviour of the vessels conformed to a sliding-filament model of muscular contraction. Using the “Maxwell model” of a muscle, the length at which the contractile element produced maximum active tension was established. The passive wall tension at this length (? 1 mN/mm) averaged only about 20% of the total wall tension the arteries were capable of producing when activated by K⁺. Under isometric conditions the K⁺-contracted basilar artery developed a maximum active wall stress of approximately 240 mN/mm². In the light of the mechanical data obtained from the length-tension measurements, the optimum resting wall tension for registration of vascular responses is discussed. It appears that the present in vitro system can be of great value in investigations of the smooth muscle function in small blood vessels.     

Rhythmic contractions in isolated small arteries of rat: role of K+ channels and the Na+, K+-pump

Small mesenteric arteries from Wistar rats display rhythmic tension oscillations, associated with oscillations in membrane potential, when stimulated with noradrenaline. The purpose of this study was to investigate the role of potassium conductance and Na⁺, K⁺-pump activity in the generation of these oscillations. The effect on the rhythmic contractions of several agents, interacting with K⁺ channels, was studied. Application of apamin, pinacidil or glibenclamide did not affect the rhythmic activity. Tetraethylam-monium (TEA) increased the frequency of the rhythmic contractions, while application of 4-aminopyridine (4-AP) increased the amplitude by approximately 50%, with no changes in frequency. Ba²⁺, on the other hand, impaired the rhythmic contractions or converted them to irregular oscillations in the presence of functional endothelium, but did not affect oscillations in endothelium-denuded vessels. Ouabain or exposure to K⁺-free solution, procedures known to inhibit the Na⁺, K⁺-pump, abolished the rhythmic contractions. This effect was immediate, suggesting that it was due to elimination of the electrogenic action of the Na⁺, K⁺-ATPase, rather than to a change in intracellular ion concentrations. Exposure to an extracellular potassium concentration of more than 20 mM also inhibited the oscillation activity. The results suggest that the oscillations are not caused by, but may be modulated by, variations in potassium conductance. The Na⁺, K⁺-pump seems to play an important role in the generation of rhythmic contractions in these vessels.     

Angiotensin II type 2 receptors facilitate reinnervation of phenol-lesioned vascular calcitonin gene-related peptide-containing nerves in rat mesenteric arteries

The present study was designed to investigate involvement of angiotensin II (Ang II) type 2 receptors (AT2 receptors) in restoration of perivascular nerve innervation injured by topical phenol treatment. Male Wistar rats underwent in vivo topical application of 10% phenol around the superior mesenteric artery. After phenol treatment, animals were subjected to immunohistochemistry of the third branch of small arteries, Western blot analysis of AT2 receptor protein expression in dorsal root ganglia (DRG) and studies of mesenteric neurogenic vasoresponsiveness. Ang II (750 ng/kg/day), nerve growth factor (NGF; 20 microg/kg/day) and PD123,319 (AT2 receptor antagonist; 10 mg/kg/day) were intraperitoneally administered for 7 days using osmotic mini-pumps immediately after topical phenol treatment. Losartan (AT1 receptor antagonist) was administered in drinking water (0.025%). Phenol treatment markedly reduced densities of both calcitonin gene-related peptide (CGRP)-like immunoreactivity (LI) and neuropeptide Y (NPY)-LI-containing fibers. NGF restored densities of both nerve fibers to the sham control level. Coadministration of Ang II and losartan significantly increased the density of CGRP-LI-fibers but not NPY-LI-fibers compared with saline control. The increase of the density of CGRP-LI-fibers by coadministration of Ang II and losartan was suppressed by adding PD123,319. Coadministration of Ang II and losartan ameliorated reduction of CGRP nerve-mediated vasodilation of perfused mesenteric arteries caused by phenol treatment. The AT2 receptor protein expression detected in DRG was markedly increased by NGF. These results suggest that selective stimulation of AT2 receptors by Ang II facilitates reinnervation of mesenteric perivascular CGRP-containing nerves injured by topical phenol application in the rat.     

Inhibitory actions of adenosine differ between ear and mesenteric arteries in the rabbit

In isolated ear and mesenteric arteries of rabbit, adenosine inhibited nerve-mediated contractions to a similar extent. However, the amplitude of the excitatory junction potentials evoked by perivascular nerve stimulation was increased by adenosine in the ear artery and decreased in the mesenteric artery. Outflows of noradrenaline and its metabolite, 3,4-dihydroxyphenylglycol, evoked by perivascular nerve stimulation were increased and decreased by adenosine in the ear and mesenteric arteries, respectively. Adenosine hyperpolarized the smooth muscle cells, by increasing potassium conductance of the membrane, with no relation to the endothelial cells. The hyperpolarizing action of adenosine was stronger in the ear artery than in the mesenteric artery. The inhibition of the nerve-mediated contraction by adenosine may be mainly due to postjunctional events in the ear artery and prejunctional events in the mesenteric artery.     

Acute and long-term effects of 17b-oestradiol on agonist-stimulated force in rat tail artery

The effects of 17β-oestradiol on the force responses to KCl and noradrenaline were investigated in rings of the rat tail artery. Incubation with 10 μm 17β-oestradiol for 100–295 min reduced the force amplitude after 5 min in high-K⁺ (140 mm ) to 10% of the control value. The inhibitory effect of the steroid was unaffected by the NO-synthase inhibitor l -NAME. Rings activated by an intermediate degree of depolarization (60 mm K⁺) were less affected by the steroid (58% of control force). The sustained force response to 1 μm noradrenaline was reduced in the presence of 17β-oestradiol to 60% of control value. Lower concentrations of 17β-oestradiol (0.1 and 1 μm ) were without acute effects on force development. However, long-term effects of 17β-oestradiol on vessel reactivity were found at these low concentrations. Rings were cultured for 3–7 days in the absence or in the presence of the steroid before they were stimulated with agonists. Cultured rings developed an increased sensitivity to noradrenaline compared with freshly prepared ones. Cocaine (30 μm) shifted the noradrenaline concentration–response curve to the left in freshly prepared rings while it had no effect in cultured ones, indicating that the increased sensitivity to noradrenaline in cultured rings depends on loss of noradrenaline uptake. Rings cultured for 7 days in the presence of 0.1 μm 17β-oestradiol developed a more pronounced supersensitivity to noradrenaline (EC₅₀ for noradrenaline was 0.13±0.03 μm in steroid exposed rings vs. 0.38±0.09 μm in control rings). Thus, prolonged treatment with 17β-oestradiol results in a potentiation of noradrenaline evoked force, in contrast to the acute effect of the steroid.     

Adrenomedullin facilitates reinnervation of phenol-injured perivascular nerves in the rat mesenteric resistance artery

Our previous report showed that innervation of calcitonin gene-related peptide (CGRP)- and neuropeptide Y (NPY)-containing nerves in rat mesenteric resistance arteries was markedly reduced by topical application of phenol, and that nerve growth factor (NGF) facilitates the reinnervation of both nerves. We also demonstrated that a CGRP superfamily peptide, adrenomedullin, is distributed in perivascular nerves of rat mesenteric resistance arteries. In the present study, we investigated the influence of adrenomedullin on the reinnervation of mesenteric perivascular nerves following topical phenol treatment. Under pentobarbital-Na anesthesia, 8-week-old Wistar rats underwent in vivo topical application of phenol (10% phenol in 90% ethanol) to the superior mesenteric artery proximal to the bifurcation of the abdominal aorta. After the treatment, the animals were subjected to immunohistochemistry of the third branch of small arteries proximal to the intestine and to vascular responsiveness testing on day 7. Topical phenol treatment caused marked reduction of the density of NPY-like immunoreactive (LI)- and CGRP-LI nerve fibers in the arteries. Adrenomedullin (360 or 1000 ng/h) or NGF (250 ng/h), which was administered intraperitoneally for 7 days using an osmotic mini-pump immediately after topical phenol treatment, significantly increased the density of CGRP-LI- and NPY-LI nerve fibers compared with saline. Treatment with adrenomedullin (1000 ng/h) or NGF restored adrenergic nerve-mediated vasoconstriction and CGRP nerve-mediated vasodilation in the perfused mesenteric artery treated topically with phenol. These results suggest that adrenomedullin, like NGF, has a facilitatory effect on the reinnervation of perivascular nerves.     

In vivo analysis of intracellular thallium-201 accumulation in skeletal muscle of the rat

The specific accumulation of the K⁺-analogue TI⁺(²⁰¹Tl⁺) in muscle after intramuscular injection was analysed by gamma spectrometry in vivo of rat hamstring muscles. A mixture (0.1ml) of ²⁰¹Tl⁺ (thallous⁺ chloride^-) and ^99mTc-pertechnetate- (Na⁺ pertechnetate^-) was given, by which ^99mTc-pertechnetate^- served as a reference substance with negligible intracellular accumulation. After 30 min 8.9 ± 5.8% of injected ^99mTc-pertechnetate^- remained in the muscle and 49+10% of ²⁰¹Tl⁺ (±SD, n = 18). The difference between ²⁰¹Tl⁺ and ^99mTc-pertechnetate- at 30 min was taken as a measure of the intracellular ²⁰¹Tl⁺ accumulation, which was 40% of the initial amount of ²⁰¹TI⁺. The half-time of the calculated intracellular ²⁰¹TI⁺ accumulation was 4.9+1.9 min. In the presence of ouabain (1.0 mM in the injectate) the intracellular ²⁰¹Tl⁺ accumulation was 25+10% (n = 7), that is ouabain decreased the intracellular ²⁰¹Tl⁺ accumulation by 38% (P = 0.0035). Non-radioactive TI⁺ (1.0 mM TI-acetate in the injectate) inhibited the uptake by 35% (P= 0.0013). Ouabain did not significantly affect the half-time for the Tl⁺ uptake. An increase in [K⁺] of the injectate from 0 to 5 mM had no significant effect. Insulin (0.2 units in the injectate) had no effect. It is concluded that the specific TI⁺-accumulating properties of muscle fibres can be studied with the present in vivo technique, which can provide information about the Na-K-ATPase activity and the membrane potential of muscle fibres.     

Angiotensin II induced contraction of rat and human small intestinal wall musculature in vitro

Background: Angiotensin II (Ang II) is a well‐known activator of smooth muscle in the vasculature but has been little explored with regard to intestinal wall muscular activity. This study investigates pharmacological properties of Ang II and expression of its receptors in small‐intestinal smooth muscle from rats and humans. Methods: Isometric recordings were performed in vitro on small intestinal longitudinal muscle strips. Protein expressions of Ang II typ 1 (AT1R) and typ 2 (AT2R) receptors were assessed by Western blot. Results: Ang II elicited concentration‐dependent contractions of rat jejunal and ileal muscle preparations. The concentration–response curve (rat ileum, EC₅₀: 1.5 ± 0.9 × 10^?8 m ) was shifted to the right by the AT1R receptor antagonist losartan (10^?7 m ) but was unaffected by the AT2R antagonist PD123319 (10^?7 m ) as well as by the adrenolytic guanethidine (3 × 10^?6 m ) and the anticholinergic atropine (10^?6 m ). Human duodenal, jejunal and ileal longitudinal muscle preparations all contracted concentration‐dependently in response to Ang II. The concentration–response curve (human jejunum, EC₅₀: 1.5 ± 0.8 × 10^?8 m ) was shifted to the right by losartan (10^?7 m ) but was unaffected by PD123319 (10^?7 m ). Both AT1R and AT2R were detected in all segments of the rat small intestinal wall musculature, whereas only AT1R was readily detectable in the human samples. Conclusion: Ang II elicits contractions of small‐intestinal longitudinal muscle preparations from the small intestine of rats and man. The pharmacological pattern and protein expression analyses indicate mediation via the AT1R.     

Characterization of voltage‐gated calcium currents in freshly isolated smooth muscle cells from rat tail main artery

The aim of the present study was to characterize voltage‐gated Ca²⁺ currents in smooth muscle cells freshly isolated from rat tail main artery in the presence of 5 mmol L^–1 external Ca²⁺. Calcium currents were identified on the basis of their voltage dependencies and sensitivity to nifedipine, Ni²⁺ and cinnarizine. In the majority of the cells studied, T‐ and L‐type currents were observed, while the remaining cells showed predominantly L‐type currents. In the latter group of cells, holding potential change from –50 to either –70 or –90 mV increased the corresponding inward current amplitude while its voltage activation threshold remained unchanged. The steady state inactivation of L‐type Ca²⁺ channels showed half‐maximal inactivation at –38 mV. A Ca²⁺‐dependent inactivation was also evident. Nifedipine (3 μmol L^–1) blocked L‐type but not T‐type Ca²⁺ currents. Ni²⁺ (50 μmol L^–1) as well as cinnarizine (1 μmol L^–1) suppressed the nifedipine‐resistant, T‐type component of the currents. At higher concentrations, both Ni²⁺ (0.3–1 mmol L^–1) and cinnarizine (10 μmol L^–1) blocked the net inward current. Replacement of Ca²⁺ with 10 mmol L^–1 Ba²⁺ significantly increased the amplitude of L‐type Ca²⁺ currents. These results demonstrate that smooth muscle cells freshly isolated from rat tail main artery may be divided into two populations, one expressing both L‐ and T‐type and the other only L‐type Ca²⁺ channels. Furthermore, this report shows that in arterial smooth muscle cells cinnarizine potently inhibited T‐type currents at low concentrations (1 μmol L^–1) but also blocked L‐type Ca²⁺ currents at higher concentrations (10 μmol L^–1).