5-hydroxytryptamine-stimulated accumulation of 1,2-diacylglycerol in the rabbit basilar artery: a role for protein kinase C in smooth muscle contraction

Cardiac transplantation and resistance artery myogenic tone

Transplantation is an effective treatment for end-stage heart disease; however, most grafts eventually fail by progressive cardiac failure. Primarily, failure is ischemic due to the occlusive nature of transplant vascular disease (TVD). Early after transplantation and preceding TVD, alterations in coronary physiology such as reduced vascular myogenic tone occur. Resistance arteries possess an inherent ability to constrict in response to transmural pressure; this constrictive response (myogenic tone) is important in fluid homeostasis. Recent evidence suggests that a decline in myogenic tone leads to deficits in cardiac contractility. Factors that reduce myogenic tone in transplantation include constitutive nitric oxide synthase and inducible nitric oxide synthase catalyzed, NO-mediated vasodilation as well as deficits in arterial contractile function. Reduced myogenic tone in allograft resistance arteries increases coronary blood flow such that hydrostatic pressure surpasses oncotic pressure, causing cardiac interstitial edema. This generalized edema decreases ventricular compliance leading to heart failure during the course of acute immune rejection of the graft. Cyclosporine A treatment reduces immune mediated dysregulation of myogenic tone, resulting in reduced interstitial edema and improved cardiac function. In this review, we discuss aspects of TVD and myogenic tone signaling mechanisms and how aberrations in myogenic regulation of arterial tone contribute to functional changes observed in cardiac transplant.

Stroke development in stroke-prone spontaneously hypertensive rats alters the ability of cerebrovascular muscle to utilize internal Ca2+ to elicit constriction

BACKGROUND AND PURPOSE

The ability of middle cerebral arteries (MCAs) to utilize intracellular smooth muscle (SM) Ca2+ to produce constriction in response to pressure and agonists was assessed in relation to hemorrhagic stroke development in Wistar-Kyoto stroke-prone (SHRSP) and stroke-resistant (srSHR) spontaneously hypertensive rats.

METHODS

MCAs were studied with the use of a pressure myograph at 100 mm Hg.

RESULTS

MCAs from srSHR and prestroke SHRSP exhibited pressure-dependent constriction and constricted in response to vasopressin or serotonin in the presence of nifedipine or the absence of [Ca2+]o. MCAs from poststroke SHRSP lost the latter functions and could only constrict in response to vasopressin/serotonin in Krebs' solution containing Ca2+ in the absence of nifedipine. This indicated that the SM could not utilize internal Ca2+ for constriction and maintained constriction by Ca2+ entry through L-type channels. The MCAs of poststroke SHRSP could not constrict to [K+]o-induced depolarization, suggesting that the agonist-induced opening of the L-type channels occurred by mechanisms other than SM depolarization. Depletion of the sarcoplasmic SM Ca2+ stores of MCAs from srSHR with cyclopiazonic acid did not prevent pressure-dependent constriction.

CONCLUSIONS

Stroke in SHRSP produced a defect in the ability of MCAs to constrict in response to vasopressin or serotonin via the use of an intracellular source of Ca2+. This could be promoted by an inability of the SM to release intracellular Ca2+, by the depletion of internal Ca2+ stores, or by a decrease in the contractile sensitivity to Ca2+ released from the internal stores.

Increased Ca2+ sensitivity as a key mechanism of PKC-induced constriction in pressurized cerebral arteries

The effects of activating protein kinase C (PKC) with indolactam V (Indo-V) and 1,2-dioctanoyl-sn-glycerol (DOG) on smooth muscle intracellular Ca2+ concentrations ([Ca2+]i) and arterial diameter were determined using ratiometric Ca2+ imaging and video edge detection of pressurized rat posterior cerebral arteries. Elevation of intraluminal pressure from 10 to 60 mmHg resulted in an increase in [Ca2+]i from 74 +/- 5 to 219 +/- 8 nM and myogenic constriction. Application of Indo-V (0.01-3 microM) or DOG (0.1-30 microM) induced constriction and decreased [Ca2+]i to 140 +/- 11 and 127 +/- 12 nM, respectively, at the highest concentrations used. In the presence of Indo-V, the dihydropyridine Ca2+-channel-blocker nisoldipine produced nearly maximum dilation and decreased [Ca2+]i to 97 +/- 7 nM. In alpha-toxin-permeabilized arteries, the constrictor effects of Indo-V and DOG were not observed in the absence of Ca2+. Both PKC activators significantly increased the degree of constriction of permeabilized arteries at different [Ca2+]i. We conclude that 1) Indo-V- or DOG-induced constriction of pressurized arteries requires Ca2+ influx through voltage-dependent Ca2+ channels, and 2) PKC-induced constriction of pressurized rat cerebral arteries is associated with a decrease in [Ca2+]i, suggesting an increase in the Ca2+ sensitivity of the contractile process.

Lipid second messengers derived from glycerolipids and sphingolipids, and their role in smooth muscle function

The processes that link activation of an external receptor to the internal mechanisms that elicit a physiological response have been the subject of extensive investigation. It has been established that rather than just being an inert barrier to protect the cell from environmental damage, there are populations of phospholipids located within the plasma membrane that act as a reservoir for signalling molecules and when a receptor binds its appropriate activating ligand a chain of events is initiated which leads to the breakdown of these lipids and the release of second messengers. Such processes are rapid enough for physiological responses to be effected. The purpose of this review is to examine the profile of lipid second messengers derived from glycerophospholipids and sphingolipids. In the former class are included phosphoinositide and phosphatidylcholine and the latter includes sphingomyelin. Hydrolysis of such parent compounds is mediated by phospholipases and the profile of metabolites appears to be agonist specific and modulated by a number of mechanisms including heterotrimeric G-protein subunits, small G-proteins, alterations in intracellular calcium concentration, protein kinase C and tyrosine kinases. The recent interest in sphingolipids, particularly in vascular smooth muscle cells, has been provoked by the observation that ceramide and sphingoid base formation is observed in response to vasoconstrictor hormones.

20-Hydroxyeicosatetraenoic acid-induced vasoconstriction and inhibition of potassium current in cerebral vascular smooth muscle is dependent on activation of protein kinase C

20-Hydroxyeicosatetraenoic acid (20-HETE), a cytochrome P450 metabolite of arachidonic acid, is a potent vasoconstrictor, and has been implicated in the myogenic activation of renal and cerebral arteries. We examined the role of protein kinase C (PKC) in the signal transduction pathway by which 20-HETE induces vasoconstriction and inhibition of whole-cell K+ current in cat cerebral vascular smooth muscle. 20-HETE induced a concentration-dependent constriction in isolated pressurized cat middle cerebral arteries (-29 +/- 8% at 1 microM). However, in the presence of an N-myristoylated PKC pseudosubstrate inhibitor peptide (MyrPsiPKC-I(19-27)), 20-HETE induced a concentration-dependent vasodilation (26 +/- 4% at 1 microM). In whole-cell voltage clamp studies, application of 20-HETE inhibited whole-cell K+ current recorded in cat cerebral vascular smooth muscle cells, an effect that was attenuated by MyrPsiPKC-I(19-27). Further evidence for the role of PKC activation in response to 20-HETE is the finding that 20-HETE increased the phosphorylation of myristoylated, alanine-rich PKC substrate in cultured cat cerebral vascular smooth muscle cells in a concentration- and PKC-dependent manner. These data provide evidence that PKC is an integral part of the signal transduction pathway by which 20-HETE elicits vasoconstriction of cerebral arteries and inhibition of whole-cell K+ current in cat cerebral vascular smooth muscle.

Intracellular Ca2+ release in flow-induced contraction of venous smooth muscle

We designed the present study to determine whether Ca2+ release from intracellular stores contributes to flow-induced contraction. We carried out experiments on segments of rabbit facial vein under isometric conditions. Intraluminal flow by perfusion of physiological salt solution (10 to 80 microL/min) caused contraction in this vessel, which was significantly inhibited by (1) 30-minute pretreatment with 10 mumol/L ryanodine, the sarcoplasmic reticulum Ca2+ channel opener, and (2) 30-minute pretreatment with concomitant application of 20 mmol/L caffeine and 1 mumol/L cyclopiazonic acid in Ca(2+)-free medium to deplete the sarcoplasmic reticulum. In comparison, contraction initiated by 300 nmol/L histamine was significantly attenuated by the same interventions. K+ (25 mmol/L)-induced contraction was unaffected by ryanodine but was reduced after depletion of the sarcoplasmic reticulum. The phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (10 mumol/L) inhibited contractions induced by flow and histamine but not by K+. These findings indicate that Ca2+ release from intracellular stores, presumably via the phosphatidylinositol pathway, contributes to flow- and histamine- but not raised K(+)-induced contractions in this vessel.

Possible mechanism to induce protein kinase C-dependent arterial smooth muscle contraction after subarachnoid haemorrhage

A possible mechanism for the induction of protein kinase C (PKC)-dependent vascular contraction independent to the increase of intracellular Ca++ was investigated in the pathogenesis of cerebral vasospasm in the double subarachnoid haemorrhage (SAH) model. The level of 1,2-diacylglycerol (DAG), which is an intrinsic PKC activator, significantly increased from days 4 to 7 in the basilar artery after the initial SAH, and the continuous administration of 1,2-bis(nicotinamido)-propane (AVS), a novel free radical scavenger, not only lowered the concentration of lipid peroxides in the CSF but also successfully suppressed the basilar arterial wall in the same model. It was suggested that lipid peroxides generated in the subarachnoid clot affect the DAG content of the cerebral artery. Analysis of hydroxy-eicosatetraenoic acids (HETEs) with high performance liquid chromatography (HPLC) revealed the production of relatively large amount of 12-HETE in the subarachnoid clot. To examine the potential effect of exogenous 12-HETE on the DAG content of the cerebral artery, the basilar artery was incubated with 12-HETE in vitro. 12-HETE induced a concentration-dependent slow increase in DAG content in the arterial wall after 6 hours of incubation. Under conditions in which DAG formation was facilitated by the Ca(++)-ionophore, DAG accumulation in the basilar artery was enhanced in the presence of 12-HETE. It was suggested that 12-HETE generated in the subarachnoid clot, induced DAG accumulation in the arterial wall by inhibition of DAG metabolism, resulting in the induction of prolonged PKC-dependent smooth muscle contraction in the pathogenesis of cerebral vasospasm.

Specific accumulation of inositol 1,4,5-trisphosphate in rabbit basilar artery in response to noradrenaline but not 5-hydroxytryptamine

This study examined the ability of 5-hydroxytryptamine and noradrenaline to stimulate inositol 1,4,5-trisphosphate (IP3) mass accumulation in segments of the rabbit basilar artery. 5-Hydroxytryptamine (5-HT, 100 microM) failed to stimulate any significant accumulation of IP3 during the 5 min period following its application. In the presence of prazosin, 5-HT (300 microM) caused a rapid, transient decrease in IP3 accumulation which was significant after 5 s but had increased to pre-stimulation levels within 15 s. In contrast, noradrenaline (10 microM) stimulated a rapid, transient accumulation of IP3 which was significant after 5 s but had declined to basal levels after 60 s. In basilar artery segments bathed in Krebs solution containing 25.7 mM K+ (normal concentration 5.7 mM), the basal IP3 concentration was significantly elevated. The IP3 accumulation stimulated by either 5-HT or raised K+ was not reduced by the presence of the alpha 1-adrenoceptor antagonist, prazosin (0.1 microM). In the presence of raised K+, 5-hydroxytryptamine caused a rapid, transient inhibition of the K(+)-induced IP3 accumulation, which was maximal after 5 s but had increased to pre-stimulation levels within 30 s in the continued presence of 5-hydroxytryptamine. Noradrenaline did not affect the IP3 accumulation induced by raised extracellular [K+]. These results provide further evidence that IP3 is not involved in 5-hydroxytryptamine-induced smooth muscle contraction in the rabbit basilar artery, but support a role for this second messenger in the contraction induced in response to noradrenaline.

Signal transduction in vascular smooth muscle: diacylglycerol second messengers and PKC action

Agonist-stimulated phospholipid turnover can generate diacylglycerol (DAG), an intracellular second messenger that activates protein kinase C (PKC). DAG can be produced from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by a phosphoinositide-specific phospholipase C and by the degradation of phosphatidylcholine (PC) by a phospholipase C or the concerted actions of phospholipase D and phosphatidate phosphohydrolase. In vascular smooth muscle, agonist-stimulated DAG accumulation is biphasic; PIP2 hydrolysis produces a transient increase in DAG, which is followed by a sustained phase of DAG accumulation from PC degradation. Metabolism of DAG attenuates PKC activation and thus results in signal termination. The metabolic fates for DAG include 1) ATP-dependent phosphorylation to form phosphatidic acid (DAG kinase), 2) hydrolysis to release fatty acids and glycerol (DAG and monoacylglycerol lipases), 3) synthesis of triacylglycerol (DAG acyltransferase), and 4) synthesis of PC (choline phosphotransferase). Hydrolysis through the lipase pathway is the predominant metabolic fate of DAG in vascular smooth muscle. Activation of PKC in vascular smooth muscle modulates agonist-stimulated phospholipid turnover, produces an increase in contractile force, and regulates cell growth and proliferation. Further research is required to investigate cross talk between signal transduction mechanisms involving lipid second messengers. In addition, spatial considerations such as nuclear PKC activation and the influence of diradylglycerol generation on the duration of PKC activation are important issues.

Importance of inositol (1,4,5)-trisphosphate, intracellular Ca2+ release and myofilament Ca2+ sensitization in 5-hydroxytryptamine-evoked contraction of rabbit mesenteric artery

1. Small strips from third-order branches of rabbit mesenteric artery (approximately 150-200 microM wide) contracted in response to noradrenaline (10 microM) or 5-hydroxytryptamine (5-HT; 10 microM) in oxygenated Krebs solution containing 2.5 mM Ca2+. In a Ca(2+)-free mock intracellular solution (0 Ca2+ plus 0.2 mM EGTA), noradrenaline (10 microM) and caffeine (10 mM) induced only a single, transient contraction in artery strips, while 5-HT (10 microM) failed to induce any response. 2. In strips of mesenteric artery which had been permeabilized with Staphylococcus alpha-toxin and bathed in Ca(2+)-free mock intracellular solution, noradrenaline (10 microM), caffeine (10 mM) and D-myo-inositol (1,4,5)-trisphosphate (IP3, 100 microM), but not 5-HT (10 or 100 microM) induced a transient contraction. In contrast to the non-permeabilized strips, contractions to noradrenaline, caffeine and IP3 were restored by prior incubation (10 min) in solution containing 0.08 microM Ca2+. The contractions to noradrenaline and IP3 in permeabilized muscle strips required the presence of 100 microM guanosine 5'-triphosphate (GTP), although in the absence of Ca2+. GTP alone did not induce contraction. 3. Exposure of permeabilized mesenteric artery strips to IP3 significantly reduced the subsequent contractile responses to caffeine. Contractile responses to caffeine and IP3 were abolished by the Ca(2+)-ATPase inhibitor, thapsigargin (1 microM). 4. Ca2+ (0.1-10 microM) induced concentration-dependent contraction in permeabilized artery strips. In strips which were submaximally contracted with 0.5 microM Ca2+/100 microM GTP, the subsequent addition of 5-HT (10 microM) stimulated further contraction. The protein kinase C inhibitor, H-7 (1 microM) abolished the 5-HT/GTP-induced contraction, but did not alter the contraction to Ca2+. 5. In non-permeabilized, endothelium-denuded segments of rabbit mesenteric artery bathed in Ca2+-replete Krebs solution, noradrenaline (10 microM) stimulated a rapid, transient accumulation of IP3. 5-HT(100 microM) failed to stimulate IP3 accumulation during exposure periods of up to 5 min. 5-HT (100 microM)did stimulate IP3 accumulation if the external K+ concentration was raised (to around 25 mM). This concentration of K+ alone did not stimulate IP3 production and the 5-HT-stimulated IP3 accumulation in the presence of elevated extracellular [K+] was abolished by the alpha l-adrenoceptor antagonist, prazosin(O.1 microM).6. These results suggest that intracellular Ca2+ release does not play an important role in 5-HT-induced smooth muscle contraction in the rabbit mesenteric artery. This is despite the fact that a significant intracellular Ca2+ pool is present in these cells, which can be discharged by either noradrenaline or IP3.However, 5-HT did stimulate smooth muscle contraction in the presence of raised intracellular calcium,suggesting that a component of the contraction to 5-HT will reflect an increase in myofilament Ca2+sensitivity, possibly due to the activation of protein kinase C.

Intracellular Ca2+ release in cerebral arteries

Vasoconstricting agonists elevate the intracellular Ca2+ concentration and induce tension development in vascular smooth muscle cells by inducing both Ca2+ influx from the extracellular space and Ca2+ release from cellular stores. The relative importance of Ca2+ release has been found to vary between different sites in the vasculature. This review examines the role of Ca2+ release in the activation of cerebral arteries produced by several vasoconstricting stimuli. Although the activation of cerebral arteries by agonists such as 5-hydroxytryptamine and noradrenaline has typically been found to have little dependence on Ca2+ release, other vasoconstrictors such as thromboxane A2, which may be released from the endothelium by other agonists, appear to induce a substantial intracellular Ca2+ release in cerebral arteries. The limited efficacy of Ca2+ influx blockers in the treatment of delayed cerebrovascular constriction occurring as a result of subarachnoid haemorrhage suggests that intracellular mechanisms such as Ca2+ release and/or the activation of protein kinase C may be important determinants of vasoconstriction under pathological conditions.

Ca2+ channel antagonists and inhibition of protein kinase C each block contraction but not depolarization to 5-hydroxytryptamine in the rabbit basilar artery

The Ca2+ channel antagonists nifedipine and verapamil each significantly inhibited (50-100%) the smooth muscle contraction induced in response to either 5-hydroxytryptamine (1 microM, 5-HT) or 20 mM K+ (K(+)-physiological salt solution) in the basilar artery. Simultaneous measurements of smooth muscle membrane potential showed that changes in potential were not modified at this time. A similar inhibitory action against the smooth muscle contraction but not the depolarization to 5-HT was obtained with the putative protein kinase C and phospholipase C inhibitors, 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (10 microM, H7) and 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (70 microM, NCDC). These data indicate that 5-HT-induced Ca2+ influx through voltage sensitive channels is important for smooth muscle contraction but not depolarization in the rabbit basilar artery.

Cerebral vasoconstrictor mediators

Endogenous cerebral vasoconstrictor mediators regulate vascular resistance and blood flow in the brain as a whole and in various regions and participate in the pathogenesis of cerebral circulatory disturbances. Vasoconstrictors are effective in the treatment of diseases associated with cerebral vasodilatation. There are variations in the response of cerebral arteries from primate and subprimate mammals; therefore, information as to similarities and differences in their response is quite important in evaluating the physiological role, involvement in pathogenesis and therapeutic usefulness of the mediators in healthy men and patients. In this review we described characteristics of the action of vasoconstrictors (amines, peptides, prostanoids, and others) on isolated cerebral arteries from mammals, including humans and monkeys.

Electrophysiology of cerebral blood vessels

In spite of the relatively large amount of in vitro and in vivo data indicating that, in a number of ways, cerebral arteries are pharmacologically different from peripheral arteries, the mechanisms responsible for these differences are far from clear. An understanding of these mechanisms is particularly important for a rational approach to the treatment of disorders of the cerebral circulation including migraine, hypertension and the responses of cerebral vessels to subarachnoid haemorrhage. This review outlines electrophysiological data which are available from cerebrovascular smooth muscle cells, including the possibility that inwardly-rectifying potassium channels, active at potentials close to the resting membrane potential, are intimately involved in the changes in smooth muscle tone which couple blood flow to regional changes in nerve cell activity. The membrane potential changes in response to perivascular nerve stimulation, noradrenaline, 5-hydroxytryptamine and endothelium-derived hyperpolarizing factor are also described, together with the underlying membrane mechanisms and their relationship to smooth muscle contraction and relaxation.

Regulation of contraction and relaxation in arterial smooth muscle

Intracellular calcium concentration ([Ca2+]i)-dependent activation of myosin light chain kinase and its phosphorylation of the 20-kd light chain of myosin is generally considered the primary mechanism responsible for regulation of contractile force in arterial smooth muscle. However, recent data suggest that the relation between [Ca2+]i and myosin light chain phosphorylation is variable and depends on the form of stimulation. The dependence of myosin phosphorylation on [Ca2+]i has been termed the "[Ca2+]i sensitivity of phosphorylation." The [Ca2+]i sensitivity of phosphorylation is "high" when relatively small increases in [Ca2+]i induce a large increase in myosin phosphorylation. Conversely, the [Ca2+]i sensitivity of phosphorylation is "low" when relatively large increases in [Ca2+]i are required to induce a small increase in myosin phosphorylation. There are two proposed mechanisms for changes in the [Ca2+]i sensitivity of phosphorylation: Ca(2+)-dependent decreases in the [Ca2+]i sensitivity of phosphorylation induced by phosphorylation of myosin light chain kinase by Ca(2+)-calmodulin protein kinase II and agonist-dependent increases in the [Ca2+]i sensitivity of phosphorylation by inhibition of a myosin light chain phosphatase. I will review the proposed mechanisms responsible for the regulation of [Ca2+]i and the [Ca2+]i sensitivity of phosphorylation in arterial smooth muscle.

Dual action of arachidonic acid on calcium mobilization in avian granulosa cells

The primary aim of this study was to evaluate the effects of arachidonic acid (AA) on calcium mobilization from intracellular compartments in digitonin-permeabilized granulosa cells isolated from the largest preovulatory follicles of laying hens. At low concentrations (ED50 0.2 microM) AA released 35% 45Ca from the endoplasmic reticulum (ER), whereas at higher concentrations (ED50 16 microM) it stimulated 45Ca efflux from mitochondria. These effects of AA were mimicked at 10-20 times lower concentration by the calcium ionophore A23187. Inositol 1,4,5-trisphosphate (IP3) also stimulated 45Ca efflux from the ER, with a markedly lower potency than AA (ED50 6.2 microM), as well as exhibiting a biphasic response. Heparin abolished the effect of IP3 and luteinizing hormone (LH), but it had no influence on AA-promoted 45Ca efflux. Moreover, the actions of IP3 and AA were additive, indicating that AA and IP3 access different Ca pools in the ER by different mechanisms. Several other unsaturated fatty acids also stimulated 45Ca mobilization from both ER and mitochondria but, with the exception of eicosapentaenoic acid, were significantly less effective than AA. It is concluded that free AA, at submicromolar concentrations that might be viewed as physiological, is a potent calcium mobilizing agent and thus may play an important role in signal transduction in avian granulosa cells, akin to that of IP3. At high (greater than 10 microM) concentrations AA removes Ca2+ from the mitochondria, an action that may be responsible for its reported inhibitory effects on steroidogenesis and other cellular functions.

Role of protein kinase C in constrictor responses of the rat basilar artery in vivo

1. The goal of this study was to determine the effects of activation and inhibition of protein kinase C on the rat basilar artery in vivo. 2. The diameter of the basilar artery was measured through a craniotomy in rats anaesthetized with pentobarbitone sodium (50 mg kg-1, I.P., supplemented with 20 mg kg-1 h-1). Diameters were measured under control conditions and during topical application of various agonists, both alone and in the presence of antagonists. 3. Serotonin (5-HT) produced concentration-related constriction of the basilar artery (baseline diameter = 234 +/- 9 microns, mean +/- S.E.M.), which was inhibited by the 5-HT2 receptor antagonist LY53857. 4. Sphingosine (10(-6) M), a protein kinase C inhibitor which binds to the regulatory site of protein kinase C, inhibited the response to 10(-8) M-serotonin (-19 +/- 2% before vs. -3 +/- 2% during sphingosine, P less than 0.05). In contrast, constrictor responses to prostaglandin F2 alpha to (PGF2 alpha; 10(-6) M) were not inhibited by sphingosine (-16 +/- 2% before vs. -18 +/- 2% during sphingosine, P greater than 0.05). 5. H-7 (10(-9) M), another protein kinase C inhibitor, which binds to the catalytic site of protein kinase C, also inhibited constriction of the basilar artery in response to serotonin, but not prostaglandin F2 alpha. 6. Phorbol 12,13-dibutyrate (PDBu, 10(-8) M), which activates protein kinase C, produced slowly developing constriction of the basilar artery. PDBu-induced vasoconstriction (-33 +/- 2%) was attenuated by sphingosine (-11 +/- 4% during sphingosine, P less than 0.05) and H-7 (-1.5 +/- 5% during H-7, P less than 0.05). 7. In summary, activation of protein kinase C appears to mediate vasoconstrictor responses of the basilar artery to serotonin, but not PGF2 alpha.