Pressure-dependent membrane depolarization in cat middle cerebral artery

Possible cellular mechanism for cerebral vasospasm after experimental subarachnoid hemorrhage in the dog

This study was undertaken to examine some of the cellular ionic mechanisms responsible for the cerebral vasospasm that occurs as a consequence of subarachnoid hemorrhage (SAH). After cisternal injection of autologous blood we documented spasm of the basilar artery upon angiography from 4 to 7 d postictus in six dogs. When these basilar arteries were isolated we observed marked membrane depolarization and enhanced electrical spike activity compared with controls. The slope of the membrane potential vs log [K]0 curve was significantly reduced in arteries exposed to SAH. Further analysis supported the concept that such altered muscle cell properties resulted from reduction in resting K+ conductance (gk). Exposure of arteries in vitro to nicorandil (10(-9)-10(-7)M) (a drug which acts by increasing gk) hyperpolarized the muscle cells and increased internal diameter. Infusion of nicorandil (3-5 micrograms/kg per min) to intact, anesthetized animals reversed, by 50%, the reduction in basilar artery diameter after experimental SAH.

Electrical stimulation of the endothelial surface of pressurized cat middle cerebral artery results in TTX-sensitive vasoconstriction

The purpose of this study was to examine the electrical and mechanical responses of cat middle cerebral arteries to electrical stimulation of the adventitial vs. intimal surface of the vessels and to determine the responses as a function of transmural pressure. Middle cerebral arteries were cannulated at both ends. Within each cannula was a stimulating electrode. Electrical stimulation (0.5-msec square current pulses at 0.5 Hz yielding 160 microA of current between electrodes) resulted in significant reduction in diameter that was greater at both 40 and 80 mm Hg vs. 100 or 140 mm Hg. Conversely, adventitial stimulation of perivascular nerves with transmural platinum stimulating electrodes resulted in significant vasodilation. The constrictor response to intimal stimulation, as well as the dilatory response to adventitial stimulation, was blocked by tetrodotoxin. The constrictor response to luminal stimulation was enhanced by scorpion toxin demonstrating a functional role for tissues containing fast Na+ channels. Perfusion with collagenase to disrupt the endothelium also abolished the constrictor response to luminal stimulation. The divergence of responses between adventitial and luminal surface stimulation may suggest that different cell layers within a blood vessel serve different functions, one to increase resistance and another to decrease resistance. For example, in cat middle cerebral arteries, the adventitial nerves (i.e., via reflexes) may increase flow, while blood-borne substances may mediate release of agents that reduce flow.

Distribution of excitatory junction potential amplitude in cat cerebral arteries: examination of its quantal nature and modulation by opiates

We used scorpion venom to release small amounts of an excitatory neurotransmitter from adventitial nerves in cat left anterior descending cerebral artery. We used glass microelectrodes to measure and record postsynaptic electrical events of minimal amplitude. These events were similar to postsynaptic spontaneous and electrically evoked excitatory junction potentials (ejp's) seen in skeletal muscle. We performed a frequency analysis of the ejp amplitudes to determine if they fit a unimodal or multimodal distribution. We also investigated the effects of phentolamine, norepinephrine, hydromorphone, and morphine on ejp amplitude and frequency in the artery. Statistical analysis of the ejp frequency and amplitude revealed a multipeaked distribution with decreasing peaks. These results were similar to the distribution reported for acetylcholine release in skeletal muscle. The ejps were inhibited by phentolamine, which suggested that these events were adrenergically mediated. Norepinephrine and the opiates, hydromorphone and morphine, reduced the frequency and amplitude of the ejp's. The vessels also constricted to increasing doses of norepinephrine both under control conditions and in the presence of opiate. These results suggest that norepinephrine blocks the ejp's by a feedback mechanism at the presynaptic membrane and that endorphins and/or enkephalins, also acting at this presynaptic site, may modulate neurotransmission in the cerebral circulation.

Early effects of tetraethylammonium chloride on the contractile properties of isolated rabbit basilar arteries

The acute vascular effects of tetraethylammonium chloride (TEA) were examined on annular segments of rabbit basilar arteries. Contractions induced by the potassium channel blocker were compared with those obtained for potassium chloride, 5-hydroxytryptamine (5-HT) and norepinephrine (NE). The greater magnitude of the contractions was of the following order: [K+] greater than 5-HT greater than TEA greater than NE. High concentrations of TEA alone (10(-2) M) generated spontaneous oscillatory contractions in cerebral vessels that were normally quiescent. Low concentrations of TEA (10(-8)-10(-6) M), which had no vasomotor properties per se, enhanced the contractile response of submaximal concentrations of 5-HT (10(-7) M) and NE (3 X 10(-6) M) and attenuated the contraction produced by 60 mM [K+]. An increased vascular response to the amines was still evident up to 3 h after the addition of TEA despite frequent rinsing with fresh buffer solutions. On arteries precontracted with TEA (10(-2) M), but not high [K+], the subsequent addition of 5-HT (10(-7) M) still induced a powerful constriction. Repeated concentration-response curves for [K+] were reproducible and, in the presence of TEA (10(-8) or 10(-6) M), the curve was displaced to the right in a competitive manner. A higher concentration of TEA (10(-4) M) was devoid of any blocking properties on the [K+]-induced response whereas, at 10(-3) M TEA, the response was potentiated, as evidenced by a shift of the curve to the left. Interactions between TEA and the cumulative response to 5-HT were difficult to interpret. Repeated exposures of the artery to 5-HT resulted in an increased maximal response with each determination (EAm = 127 +/- 9% and 149 +/- 14% of control values following the second and third applications, respectively). With TEA (10(-6) M), the increase in the maximal contractile effect noted previously was not observed. Contractions induced by single concentrations of TEA (10(-2) M) or [K+] (60 mM) were calcium dependent, were abolished completely in a calcium-free medium, and were depressed by the calcium antagonist nimodipine. 5-Hydroxytryptamine-induced contractions (10(-5) M) were less sensitive to withdrawal of calcium from the extracellular medium (31 +/- 6% relative to the maximal response at 4 mM calcium). Hence, an acute reduction in potassium conductance in cerebrovascular smooth muscle produced by TEA has complex, concentration-dependent effects and reproduces only part of the spectrum of effects of cisternal injection of blood on cerebrovascular reactivity.

The effect of alkaline pH and transmural pressure on arterial constriction and membrane potential of hypertensive cerebral arteries

These studies were undertaken to examine the effect of alkalosis to modify "pressure-induced" activation of isolated cerebral arteries from spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto (WKY) controls. At pH 7.4 and PCO2 of 34 torr elevation of transmural pressure from 0-140 mm Hg resulted in myogenic activation preceded by membrane depolarization in both SHR and WKY. The degree of developed myogenic tone in SHR was elevated above WKY. Alkalosis (pH 7.4-7.7) depolarized and activated SHR cerebral arteries to a greater extent than WKY. Furthermore, both the electrical and mechanical responses to elevation in transmural pressure were exaggerated in SHR compared to WKY at pH 7.7 (PCO2 constant at 34 torr). Manipulation of PCO2 at constant pH of 7.4 had similar effects on "pressure-induced" myogenic tone in both SHR and WKY. Thus, cerebral arteries from both SHR and WKY depolarize and develop myogenic tone in response to increasing transmural pressure. This response is augmented in SHR, but to a much greater extent upon elevation of extracellular pH, while PCO2 is maintained within normal limits. The implications of these findings are discussed.

Evidence that intraluminal pressure affects high potassium- and serotonin-induced contractions differently in the bovine middle cerebral artery: an in vitro study

The effects of changing intraluminal pressure on contractions induced by 70 mM potassium (K+) and 10(-7), 10(-6), and 10(-5) M serotonin (5-HT) were studied in vitro in bovine middle cerebral arteries. Changes in vessel outside diameter in whole-mounted cylindrical sections of artery were detected with a photoelectric infrared device. High K+-or 5-HT (10(-5)M)-induced contractions peaked at 25 mm Hg and were significantly correlated with increasing intraluminal pressure between 25 and 175 mm Hg. Contractions induced with lower concentrations of 5-HT (10(-6), 10(-7) M), norepinephrine, and histamine peaked at 75 mm Hg but were not significantly correlated with rising pressure. Phentolamine (2 X 10(-6) M) added to the extraluminal bath had negligible influence on pressure's ability to affect K+- and 5-HT-induced contractions differently. Reducing bath temperature to 27 degrees C reduced the K+ response at each pressure, but similar temperature changes had little affect on the 5-HT-induced contractions. The K+ response became less sensitive to increasing pressure at low temperatures. Nifedipine (10(-7) M) almost totally eliminated K+-induced contractions, while significantly reducing the responses to all concentrations of 5-HT. The 5-HT responses appeared more sensitive to increasing intraluminal pressure in the presence of nifedipine. Maximum Ca++-induced contractions in the presence of 10(-5) M 5-HT and high K+ occurred at 25 mm Hg, while Ca++-induced contractions and Ca++-induced contractions in the presence of 10(-7) 5-HT or K+ plus 5-HT were maximum at 75 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Pressure-induced myogenic activation of cat cerebral arteries is dependent on intact endothelium

These studies were designed to determine the role of cerebral vascular endothelium in the "myogenic" depolarization and contraction observed in isolated cat middle cerebral arteries exposed to high transmural pressures. With intact endothelial cells we observed, on elevation of transmural pressure in cannulated isolated arteries, significant membrane depolarization, action potential generation, and reduction in internal diameter. After perfusion of the same vessels with collagenase and elastase for short periods of time to disrupt the endothelial layer, all previous responses to elevation of transmural pressure were no longer seen. Even though enzyme perfusion had no effect on membrane potential at "control" levels of transmural pressure, it abolished the pressure-dependent depolarization, action potential generation, and constriction. Furthermore, the contractile response to agonist stimulation was maintained after endothelial disruption via enzymes, showing that this method of endothelial disruption did not appreciably damage muscle cells. The data document a dependence of an intact endothelium in mediating the activation of isolated cat cerebral arteries in response to a changing transmural pressure. Thus, it is possible that the endothelial cell may serve as a transducer in the autoregulatory response to pressure.

Effect of reduced oxygen availability upon myogenic depolarization and contraction of cat middle cerebral artery

The goal of this study was to determine whether electrophysiological mechanisms contribute to the relaxation of cat middle cerebral artery in response to decreased ambient Po2 and whether decreased Po2 alters the myogenic depolarization and contraction of this vessel in response to elevations in transmural pressure. In one series of experiments, arterial segments (200-500 micron outer diameter) were isolated and mounted in an in vitro tension transducer to allow continuous measurement of active tension as bath Po2 was reduced. In these experiments, vessel relaxation occurred primarily between 150 mm Hg Po2 and 40 mm Hg Po2, suggesting that cerebral arteries are sensitive to alterations of Po2 in the physiological range. Relaxation did not result from the activation of dilator nerves in the vessel wall, since it was unaffected by tetrodotoxin. Arterial segments were also cannulated with micropipettes and subjected to elevations in transmural pressure during 300 mm Hg Po2 and 50 mm Hg Po2 superfusion. During 300 mm Hg Po2 superfusion, cannulated vessels exhibited myogenic depolarization and maintained their diameter as transmural pressure was increased; 50 mm Hg Po2 superfusion inhibited spontaneous spike activity, decreased the slope of the myogenic depolarization, and partially inhibited vessel contraction in response to elevated transmural pressure. These effects are independent of the parenchymal cell environment and appear to be mediated, at least in part, by electrophysiological mechanisms.

A membrane electrical mechanism for hypoxic vasoconstriction of small pulmonary arteries from cat

Using specially fabricated muscle myographs, we examined electrical and mechanical responses to reduction of PO2 in small (less than 300 micron) pulmonary arteries excised from cat lungs. Upon lowering PO2 from 400 to 50 mm Hg, these preparations consistently developed contractile responses concomitant with membrane depolarization and action potential generation. The largest changes in electromechanical responses to reduction of PO2 occurred between 150 and 50 mm Hg. These data strongly suggest that hypoxic activation of small pulmonary arteries is mediated by direct effects of reduced PO2 on muscle cell membrane ionic conductance systems.

Enhanced myogenic depolarization in hypertensive cerebral arterial muscle

We have previously demonstrated pressure-dependent membrane depolarization and action potential generation in cat cerebral arteries. It was the purpose of this study to examine and compare the membrane electrical responses to increasing transmural pressure in spontaneously hypertensive rats with those of their normotensive Wistar-Kyoto controls. It was found that at transmural pressures from 40-160 mm Hg, spontaneously hypertensive rat cerebral arterial muscle depolarized more than normotensive counterparts. Pressure-induced action potentials could be recorded from arterial segments from both animal strains; however, the amplitude and upstroke velocity was significantly greater in spontaneously hypertensive rat cerebral arterial muscle. These data suggest that there are altered ionic permeabilities in spontaneously hypertensive rat cerebral arterial muscle which result in enhanced response to increasing transmural pressure. The implications of these findings are discussed.

Cellular actions of halothane on cat cerebral arterial muscle

The effects of halothane on intracellular membrane potential (Em) and force development in cat MCA were studied. Halothane (0.07-0.14 mM/1) relaxed isolated MCA which had developed myogenic tone. Measurement of Em showed that halothane depolarized this preparation in a dose-dependent fashion in the face of vessel relaxation, demonstrating uncoupling of electrical and mechanical activity. Halothane markedly inhibited the contractile effects of histamine and serotonin suggesting that, apart from its direct action on cerebral arterial tone, it also blunts the action of vasoactive agents. When this preparation is partially depolarized from -62 to -50 mV with excess K+, halothane, while having only a small (1.2 mV) additional depolarizing effect, consistently elicits contraction rather than relaxation. Thus, the action of this particular volatile anesthetic on cerebral arteries can depend upon the resting level of Em. These studies indicate that halothane relaxes myogenic tone in cat MCA by an intracellular mechanism, but that the direction of its effect (i.e., relaxation vs. contraction) may depend upon the prior level of Em and muscle cell activation.

In vivo membrane potentials of smooth muscle cells in the caudal artery of the rat

Membrane potentials measured in vivo may differ significantly from those measured in vitro in part due to humoral factors, innervation, and wall tension. These studies were initiated to determine whether it is feasible to record membrane potentials from vascular smooth muscle cells in vivo in the caudal artery of the pentobarbital-anesthetized male Wistar rat. Membrane potentials were measured using glass microelectrodes and correlated with systolic, diastolic, and mean blood pressures. For systolic blood pressures between 100 and 140 mmHg the average resting membrane potential was -38.4 +/- 0.48 mV. There was good correlation of systolic, diastolic, and mean blood pressures with membrane potential between 100 and 140 mmHg (r = 0.89, 0.75, and 0.89, respectively). Below 80 mmHg the arterial muscle cells became more depolarized than would be expected if the membrane potential were determined solely by transmural pressure. The depolarized membrane potential at low arterial pressures may be due to enhanced neural input. Spontaneous electrical activity was observed in some of the in vivo cells. When action potentials were present, they were generated at rates between 1-2/s and 6-7/min. These studies indicate that it is feasible to measure membrane potentials from arterial smooth muscle cells in vivo in the caudal artery of the rat.