Effect of local anesthetics on plasma protein secretion by rat hepatocytes

The effects of some local anesthetics on plasma protein secretion by rat liver slices have been studied and have been compared with those of colchicine. Rat liver slices were pulse-labelled with l-[¹⁴C]leucine for 9 min at 37°C, collected on filter paper, washed with non-radioactive leucine and reincubated in the presence or absence of the drug to be tested. The radioactive plasma proteins produced were obtained by immunoprecipitation from either the chase medium or from the washed slices. Chlorpomazine, (3 · 10^?5 M), dibucaine (10^?5 M), lidocaine (10^?3 M) and procaine (5 · 10^?5 M) inhibited both the synthesis and secretion of plasma protein but did not affect the uptake of l-leucine into the slices nor the incorporation of phosphate into intracellular nucleotide phosphates or into phopholipids. The inhibition of secretion elicited by these drugs is probably not due to the inhibition of protein synthesis since cycloheximide, when added to the chase medium at a concentration which completely inhibits protein synthesis, did not inhibit plasma protein secretion, while cycloheximide plus procaine did inhibit secretion and also caused a retention of non-secreted plasma proteins within the slices. Unlike colchicine, howover, procaine did not cause the retained plasma proteins to accumulate in Goli-derived secretory vesicles, but showed a more general effect causing a distribution among several cell fractions.     

Prostaglandins inhibit testosterone secretion by mouse testes in vitro

Production of testosterone (T) by decapsulated mouse testes in vitro was significantly inhibited by adding prostaglanain (PG) A₁, PGA₂ or PGE₁ to the incubation medium. Prostaglandin A₁ at a concentration of 10^?6M inhibited T production in this system both in the presence of moderate amounts of hCG (12.5 or 25.0 mIU/ml), and in the absence of gonadotropins. However, in the presence of very high levels of hCG (125.0 mIU/ml), all PGs tested appeared to have had a slight potentiating effect on T production when added in concentrations ranging from 10^?7 to 10^?5M, and the inhibition of T accumulation in the medium was consistently observed only when the concentration of PGs was increased to 10^?3M. These results suggest that a direct effect of PGs on testicular steroidogenesis may account for, or contributes to, the decrease in peripheral T levels observed after administration of PGs in vivo.     

The effect of serum,EGF, PGF2α and insulin on S6 phosphorylation and the initiation of protein and DNA synthesis

To test the connection between S6 phosphorylation and the activation of protein and DNA synthesis, we compared the effects of serum, epidermal growth factor (EGF), prostaglandin F_2α (PGF_2α) and insulin (which is not mitogenic in these cells). Increasing concentrations of serum or EGF produced roughly parallel effects on all three processes, though the maximum response elicited by EGF (10^?9 M) was only a portion of that caused by saturating levels of serum (7.5% to 10%). PGF_2α (8.5 × 10^?7 M) alone acted similarly to EGF (10^?9 M) and with EGF produced a synergistic effect on all three processes. Insulin (10^?9 M) alone stimulated both S6 phosphorylation and protein synthesis to approximately the same level as EGF or PGF_2α, but had no effect on initiation of DNA synthesis. Thus neither stimulation of S6 phosphorylation nor activation of protein synthesis is sufficient for initiation of DNA synthesis. The requirement for S6 phosphorylation could not be dissociated from the activation of protein synthesis. Ribosomes containing the most highly phosphorylated forms of S6 appear to have a selective advantage in entering polysomes.     

Hydrocortisone binding receptor in the rat pituitary GH3 cell line.

A receptor with specificity and high affinity for hydrocortisone (HC) has been found in the cytosol of GH₃ cells, a growth hormone (GH) producing culture. Scatchard analysis indicated that the interaction of [³H]HC with the receptor has an apparent dissociation constant (K_d) of about 6.0 × 10^?9M and a concentration of binding sites of approx. 1 × 10^?13 mol/mg cytosol protein. The second order association rate constant was determined to be 1.5 × 10⁶ M^?1 min^?1. The receptor activity is stable at 2°C for several hours, but is destroyed completely by heating at 37°C for 1 hour, or by treatment with pronase, only partially by RNase, but not by DNase. The binding of [³H]HC to the cytosol receptor is inhibited by unlabeled progesterone (PR) or dexamethasone to the same extent as the inhibition by unlabeled HC. However, it is only partially inhibited by testosterone, 17-methyl-testosterone, 17α and 17β-estradiol, and 4-pregnen-20β-ol-3-one, and is unaffected by 5α-pregnan-3β,20β-diol. The biological role for these receptors in the regulation of GH synthesis is supported by the observations that the HC-stimulated production of GH is antagonized by PR, which competes with the binding of HC to the receptor.     

Effects of colchicine and 2-Br-alpha-ergocryptine-methane-sulfonate (CB 154) on the release of prolactin and growth hormone by functional pituitary tumor cells in culture

The effects of colchicine and 2-Br-α-ergocryptine-methane-sulfonate (CB 154) on the release of prolactin and growth hormone have been studied in a clonal strain of rat pituitary tumor cells (GH₃) in monolayer culture. These cultures produce both prolactin and growth hormone and release both proteins spontaneously into the medium without storing them in large amounts. Immunological methods were used to measure both intracellular and extracellular concentrations of the hormones. Colchicine (5 × 10^?6 M for 3 hours) caused a 2- to 3-fold increase in intracellular concentrations of prolactin and growth hormone but, under basal conditions, had little or no measurable effect on the amounts of hormone accumulated in the medium during the course of the standard three hour treatment period. This latter finding evidently is due to a lag in the onset of drug action. Colchicine had little or no effect on accumulation of extracellular prolactin during the first two hours of treatment whereas such accumulation was depressed by over 60% during the third hour of treatment. Previous studies have shown that treatment of GH₃ cells with thyrotropin releasing hormone (TRH) and hydrocortisone (HC) increases both intra and extracellular levels of prolactin and growth hormone, respectively. In cultures treated with TRH (5 × 10^?8 M), colchicine (5 × 10^?6 M for 3 hours) increased intracellular prolactin by about 70% and decreased extracellular hormone by 10%. In cultures treated with HC (3 × 1O^?6 M), colchicine increased intracellular growth hormone by more than 100% and decreased medium concentrations of the hormone by 15%. Colchicine did not significantly alter total hormone (intracellular + extracellular) accumulation, cellular uptake of ³H-amino acids, or total cell protein synthesis. The synthetic ergot alkaloid, CB 154, (3.3 × 10^?6 M for 3 hours) caused an 80% increase in intracellular, and a nearly 50% decrease in extracellular, prolactin without affecting the accumulation of growth hormone, the uptake of ³H-labeled amino acids, or overall protein synthesis in the cultures. Elevation of medium potassium concentration from a basal value of 5.3 mM to 3–5 × 10^?2 M (by addition of KCl) decreased intracellular levels of prolactin by 85% and growth hormone by 55%. These effects of high potassium were blocked by colchicine and by CB 154. We conclude that colchicine, after a lag period of two hours, acts to inhibit the release of prolactin and growth hormone from GH₃ cells. By the end of three hours of treatment, this inhibition is over 60% complete in the case of prolactin. The qualitatively different effects of colchicine and CB 154 on prolactin and growth hormone release suggest that these two secretory blocking agents probably act on GH₃ cells by different mechanisms.     

Differential effects of cAMP and cGMP on in vitro epidermal cell growth.

Primary keratinocyte cultures free of dermal fibroblasts were used to investigate the effect of varying cyclic AMP (cAMP) concentrations on epidermal cell function. Addition of 10^?3, 10^?4 or 10^?5 M dibutyryl cAMP to plated cells (day 1) results by day 5 in a dose dependent increase of [³H]TdR incorporation into DNA as determined by increases in both the labeling index and incorporation of ³H label into an isolated DNA fraction. 8-Bromo cAMP, another cAMP analogue, likewise induced keratinocyte proliferation. The proliferative response was dose and time dependent, and 5- to 6-fold increases in ³H label incorporated into DNA were seen at day 6, 8 and up until day 15 of culture. Moreover, elevation of cellular cAMP by addition of cholera toxin, an irreversible stimulator of adenylate cyclase, also demonstrated a time dependent stimulation of [³H]TdR uptake into DNA and increased the labeling index. Specific histochemical staining for keratinaceous protein (Kreyberg technique) demonstrated that elevated cAMP levels also enhance the production of specialized (differentiated) epidermal cells. Determination of the level of cAMP and cyclic GMP (cGMP) by RIA of partially purified fractions of the cultures revealed that addition of 8-bromo cAMP or cholera toxin to the cultures increased the levels of cAMP but not of cGMP. Addition of 8-bromo cGMP to the keratinocytes on day 1 at concentrations of 10^?6, 10^?7 or 10^?8 M had no effect on culture proliferation on days 4, 6 and 8, although qualitative changes in the electron microscopic pattern of the culture stratification and specialization were observed. The results indicate (1) both large and moderate increases in cellular cAMP levels induce keratinocyte culture proliferation and specialization in the absence of fibroblasts or dermal influences, (2) the quantitative enhancement of keratinocyte growth and specialization occurs without apparent participation of cGMP, (3) cGMP may be a qualitative effector of epidermal cell differentiation.     

Acetylcholine potentiation of field stimulated rat vas deferens: A pre-synaptic muscarinic mechanism?

Acetylcholine (ACh) was found to markedly enhance the nerve stimulation induced twitch response of isolated, field-stimulated rat vas deferens (RVD). The ED₂₀₀ (concentration which enhances the twitch response to 200% of control) for this potentiation was 6 × 10^?6M with the maximum twitch response being increased by more than 3 fold (325 ± 30%). Carbachol (ED₂₀₀ = 8.5 × 10^?7M) showed identical results. With each drug the potentiation was competitively antagonized by atropine (10^?7?10^?5M). Physostigmine 10^?8?10^?6M) both enhanced the basal twitch response (215 ± 8% of control at 10^?5M) and the sensitivity of the RVD to ACh (ED₂₀₀ = 3.3 × 10^?7M) but not to carbachol. Atropine, on the other hand reduced the basal twitch response by 18 ± 3% at 10^?5M. Hemicholinium (10^?4M) also reduced the basal twitch responses by 23 ± 5%. ACh (10^?7M?10^?5M) did not modify the responses of unstimulated RVD to norepinephrine or KCl suggesting a pre-synaptic site of action. Taken together these results are compatible with the presence of a pre-junctional, excitatory muscarinic mechanism in the field stimulated RVD. That this cholinergic system may be of physiological significance is supported by the observations that atropine and hemicholinium depress while physostigmine enhances the twitch response in the absence of exogenous ACh.     

Metabolic sequestration of putrescine in Neurospora crassa

The metabolic fate of putrescine labelled $\text{in}\text{vivo}$ was investigated after administration of a trace (10^?7 M) of L-[¹⁴C]ornithine to exponentially growing mycelia of $\text{Neurospora}\text{crassa}$, followed by a large chase (2 mM) of L-[¹²C]ornithine. The specific radioactivities of putrescine and spermidine were determined during the chase period by reaction with [³H]dansyl chloride of known specific radioactivity and isolation of the dansyl-derivatives by thin-layer chromatography. Radioactivity remained in the putrescine pool for over 2 h during the chase period. This suggests that putrescine is largely sequestered (80% or more) $\text{in}\text{vivo}$. The metabolic sequestration of polyamines may be a significant factor in the regulation of polyamine synthesis.     

Effect of prolonged prostaglandin exposure on prostaglandin synthesis by human lung fibroblasts

Pretreatment of human lung fibroblasts with PGE₂ but not PGF_2α enhanced synthesis of prostaglandins (PGs). The effect of the pretreatment on PG synthesis was related to the concentration of PGE₂ that was added to the culture medium. Pretreatment with PGE₂ at 5 × 10⁻¹²M did not enhance PG synthesis whereas pretreatment with PGE₂ at 5 × 10⁻⁶M induced a maximal effect. Production of PGs was increased following 1 day of pretreatment with PGE₂ and was increased further following 3 days of pretreatment. The PGE₂ treated cells showed only a slight increase in the bradykinin-induced release of radioactivity from cells prelabeled with [³H]arachidonic acid but showed a dramatic increase in the bradykinin-induced synthesis of radio-labeled PGs. The conversion of free arachidonate to PGs in both intact cells and in a cell-free preparation was increased by PGE₂ pretreatment. The presence of cyclohexamide during the pretreatment did not inhibit the PGE₂-induced activation of PG synthesis. Taken together, the results indicate that pretreatment of cells with PGE₂ increased PG synthesis by augmenting the conversion of arachidonate to PGs.     

Presynaptic Control of the Synthesis and Release of Dopamine from Striatal Synaptosomes: A Comparison Between the Effects of 5-Hydroxytryptamine, Acetylcholine, and Glutamate

The effects of 5-HT and glutamate on dopamine synthesis and release by striatal synaptosomes were investigated and compared with the action of acetylcholine, which acts presynaptically on this system. 5-HT inhibited (28%) synthesis of [¹⁴C]dopamine from L-[U-¹⁴C]tyrosine, at 10-⁵M and above. This contrasts with the action of acetylcholine, which stimulated [¹⁴C]-dopamine synthesis by 24% at 10-⁴ M. Tissue levels of GABA were unaffected by either 5-HT or acetylcholine up to concentrations of 10-⁴ M. The inhibitory action of 5-HT (5 × 10^?5 M and 2 × 10^?4 M) on [¹⁹C]dopamine synthesis was completely abolished by methysergide (2 × 10^?6 M). Higher concentrations of methysergide (10^?4 M) or cyproheptadine (10^?5 M) inhibited [¹⁴C]dopamine synthesis by 28% and 25%, respectively, when added alone to synaptosomes. However, only methysergide prevented the further inhibition of synthesis caused by 5-HT. At concentrations of 2 × 10^?5 M and above, 5-HT stimulated [¹⁴C]dopamine release. This releasing action differed from that of acetylcholine, which occurred at lower concentrations (e.g., 10^?6 M). Methysergide (up to 10^?4 M) or cyproheptadine (2 × 10^?4 M) did not reduce the 5-HT (5 × 10^?5 M)-induced release of [¹⁴C]dopamine, but methysergide (10^?4 M) showed a potentiation (49%) of this increased release. The stimulatory effects of 5-HT (2 × 10^?5 M) and K⁺ (56 mM) on [¹⁴C]dopamine release were additive, indicating that two separate mechanisms were involved. However, when both agents were present the stimulatory effect of K⁺ (56 mM) on [¹⁴C]dopamine synthesis was not seen above the inhibitory effect of 5-HT. Glutamate (0.1-5 mM) did not affect [⁴C]dopamine release or its synthesis from L-[U-¹⁴C]tyrosine. It is concluded that 5-HT modulates the synthesis of dopamine in striatal nerve terminals through a presynaptic receptor mechanism, an action antagonised by methysergide. The releasing action of 5-HT apparently occurs through a separate mechanism which is also distinct from that involved in the response to K⁺ depolarisation.     

Enhanced synthesis and extracellular accumulation of hyaluronic acid during stimulation of quiescent human fibroblasts by mouse epidermal growth factor

The effect of mouse epidermal growth factor (mEGF) on the synthesis of glycosaminoglycans and glycoproteins by human fibroblasts has been studied. The addition of physiological concentrations (10^?9 M) of mEGF to quiescent cultures preincubated in the absence of serum was found to elicit an increased incorporation of ³H-glucosamine into the glycosaminoglycans and glycoproteins of both the cellular and extracellular fractions. Although the growth response to the factor, as measured by DNA replication, was minimal under these conditions as compared with the effect of serum, the mEGF-induced incorporation of glucosamine into these cellular constituents and into the extracellular glycoproteins was comparable to that elicited by serum shift-up. Serum, however, caused a significantly larger incorporation of glucoasmine into extracellular, acid-soluble glycosaminoglycans, which were shown to contain hyaluronic acid as the major component. As previously demonstrated, the growth response to mEGF can be enhanced several fold by an mEGF-binding arginine esterase, which is normally associated with the factor in vivo, and by ascorbate. The esterase was found to increase markedly the mEGF-induced incorporation of glucosamine into extracellular hyaluronic acid, while the addition of ascorbic acid did not significantly alter glucosamine incorporation.     

The Effect of Phosphinothricin (Glufosinate) on Glutathione Synthesis in Plants

The inhibitory effect of DL-phosphinothricin (glufosinate) on glutathione synthesis was studied in vivo and in vitro. The influence of phosphinothricin on γ-glutamylcysteine synthetase was compared with the already known effects of l -buthionine sulfoximine and l -methionine sulfoximine. The results showed that phosphinothricin and buthionine sulfoximine are inhibitors of γ-glutamylcysteine synthetase of plants. With both substances the enzyme was inhibited by 50 % at a concentration of 7 . 10^?4M (pI₅₀ = 3.15). Methionine sulfoximine reduced the enzyme activity by 50% at 5 . 10^?2 M (pI₅₀ = 1.30). It is discussed that the target enzyme of phosphinothricin is the glutamine synthetase whereas the γ-glutamylcysteine synthetase is only an accessory target.     

A novel response to propranolol: Contractile response in the isolated rabbit ear artery

Propranolol caused a contractile response in the isolated rabbit ear artery (EA). The concentration of propranolol causing a threshold contraction was 1.76 × 10^?6M while that causing a maximal contraction of 2.2 ± 0.18 g was 3 × 10^?5M. Higher concentrations caused tissue relaxation. Phentolamine, 10^?7 and 10^?6M reduced the propranolol-induced contractions by 50% and 90%, respectively while prazosin, 10^?8, 10^?7 and 10^?6M caused reductions of 54, 74 and 88%, respectively. Reserpinization of the rabbit with 5 mg/kg 24 hours before use eliminated the EA contractile response to tyramine but had no effect on that to propranolol. Desmethylimipramine plus deoxycorticosterone acetate enhanced the submaximal contraction of the EA to propranolol. $\text{In vitro}$ denervation with 6-hydroxydopamine (6-OHDA) decreased the response of the EA to tyramine and propranolol by 96% and 85% respectively but increased that to norepinephrine (NE) by 11%. Rabbit thoracic aorta (TA) did not respond to propranolol. In EA contracted with vasopressin o or 30 mM potassium, propranolol 10^?4 and 3 × 10^?4M caused a 20% and 100% relaxation, respectively. It is concluded that propranolol elicits a contractile response in the EA, at least in part, by direct activation of postsynaptic alpha adrenoceptors.     

The effects of biogenic phosphates on proteinase-induced protein cleavage and functioning of plasminogen activator

We showed, using the method of lysis of fibrin plates and five substrate proteins in a thin layer of agar gel, that inorganic orthophosphate (0.001–0.06 M) enhances by 50–250% the activatory functions of streptokinase, urokinase, and tissue plasminogen activator and, in general, by 1.2–12.0 times enhances protein lysis by trypsin, α-chymotrypsin, subtilisin, papain, bacterial metalloprotease, and even pepsin at a concentration < 4 mM. At higher concentrations, phosphate sharply inhibited pepsin activity and inhibited by 40–50% gelatin lysis by papain and gelatin (at a peak concentration) and casein lysis by metalloprotease. Inorganic pyrophosphate ions at concentrations of 10^?8–10^?1 M enhanced the cleavage of a number of proteins by serine proteinases and, at concentrations of 10^?5–10^?3 M, the activities of pepsin, plasminogen tissue activator, and streptokinase by 100 and 40%, respectively. The pyrophosphate concentrations of >10^?3 and >10^?4 M inhibited pepsinand metalloproteinase-catalyzed lysis of vritually all proteins. ATP increased casein lysis by serine proteinases, metalloproteinase, and pepsin by 20–60% at concentration of >10^?3 M and by 30–260% at 10^?2 M concentration. At concentrations of 10^?2 M, it inhibited the cleavage of some proteins by trypsin, chymotrypsin, papain, and metalloproteinase by 20–100%, and, at concentrations of 10^?3 M, lysis of albumin by pepsin and other proteins (except for fibrinogen) by metalloproteinase. A GTP concentration of 10^?7–10^?2 M increased protein degradation by serine proteinases, papain, and gelatin lysis by pepsin by 20–90%, whereas albumin lysis was inhibited by 40–70%. The presence of 10^?6–10^?5 M GTP led to a slightly increased degradation of hemoglobin and casein by bacterial metalloproteinase, while ≥10^?3 M GTP induced a drop in the activity of the metalloproteinase by 20–50%. ADP enhanced gelatin lysis by trypsin, casein lysis by pepsin and papain, and inhibited metalloproteinase activity by 20–100% (at ≥10^?3 M). Peculiarities of the effects of AMP and GD(M)P on gelatin lysis were found.     

Effect of Gibberellic Acid on the Elongation Rate of Agrostis alba Root Hairs

The influence of gibberellic acid over a wide range of concentrations on the rate of elongation of root hairs of redtop grass was investigated. The rate of root hair elongation was increased by GA over the concentration range of 10^?7 to 10^?12 M inclusive, with peak stimulation occurring at 10^?6 M. Although root hair growth was slightly accelerated by 10^?6 M GA, this concentration damaged many root hairs and caused some to stop growing altogether. Rate of root hair elongation was reduced to less than 84% of the control by 10^?5 M GA.     

Studies on the effects of insulin and acetylcholine on activation of glycogen synthase and on glycogenesis in hepatocytes

The addition of insulin (4.0 × 10^?11 M) or acetylcholine (10^?6 M) to isolated hepatocytes stimulated glycogen accumulation and this stimulation was more pronounced when the medium glucose was raised from 50 to 300 mg percent. Studies with [¹⁴C]-glucose showed a two-fold stimulation in glycogen synthesis by the addition of insulin (4.0 × 10^?11 M) or acetylcholine (10^?6 M). A sixteen percent increase in the activity of glycogen synthase was observed in cells incubated for 10 minutes with insulin (4.0 × 10^?11 M) or acetylcholine (10^?6 M), whereas at one hour incubation a 40 percent increase in activity was observed with the same concentration of insulin or acetylcholine. The effects of insulin and acetylcholine were not additive.     

An inhibitory effect of arachidonic acid on subsequent responses of canine cerebral arteries to serotonin and other agonists

Arachidonic acid (AA) at 10^?4M and 10^?3M produced a phasic contraction in isolated canine basilar arteries that peaked rapidly and then slowly declined. This contraction was evidently due to the conversion of AA to prostanoids because it was blocked by cyclooxygenase inhibitors and because 11, 14, 17 eicosatrienoic acid (10^?3M), which is not a cyclooxygenase substrate, failed to produce a contraction. When the artery was exposed to 10^?3M AA for 20 min and washed, subsequent contractile responses to 10^?6M serotonin (5-HT) were only 10% of control. Contractions produced by prostaglandin E₂ (10^?5M), uridine triphosphate (10^?4M) and potassium (5.5×10^?4M) were inhibited to a lesser degree than 5-HT, the response to potassium being the least affected (66% of control). This damaging effect of 10^?3M AA did not occur if the artery was washed at peak contraction nor with 10^?4M AA. Autooxidation products were evidently not responsible for the damage because prior oxygenation (90 min) of 10^?4M AA had no such effect. Pretreatment with superoxide dismutase or ascorbate did not prevent the inhibition, suggesting that free radical reactions were not involved. Pretreatment with indomethacin (3×10^?4M) or meclofenamate (10^?4M) also failed to prevent the inhibitory phenomenon. Saponin, a detergent, produced similar inhibitory effects but 11, 14, 17 eicosatrienoic acid or oleate (10^?3M) did not. The arteries partially recovered from the inhibition with time. In conclusion, AA produced contraction in basilar arteries by inducing prostaglandin synthesis but can produce secondarily by an unidentified mechanism an inhibition of the contractile responses evoked by various agonists that is both time and concentration dependent.     

Interaction between phenylephrine and prostaglandins E1 and F1α on the contractile responses of vascular muscle

Prostaglandins (PGs) E₁ or F_1α (1.4−8.4 × 10⁻⁸ M) contracted strips of rabbit aorta and increased the contractions produced by 1−6 × 10⁻⁷ M phenylephrine (PE). The addition of the PGs simultaneously with PE or after a low concentration of PE (2 × 10⁻⁷ M) significantly increased the PE-induced contractions. However, when the PGs were added after a higher concentration of PE (6 × 10⁻⁷ M) an additional increase in the PE-induced contraction was produced with PGF_1α but not with PGE₁. Isobolic plots of the data obtained from the simultaneous addition of PE and the PGs indicate that both PGs interact with PE in a synergistic or potentiative manner, suggesting that their effects are mediated through different receptor mechanisms. Addition of the PGs after a high dose of PE indicates that there may also be either qualitative or quantitative differences between PGE₁ and PGF_1α.     

Effects of prostacyclin on the canine isolated basilar artery

Prostacyclin (PGI₂) produced a biphasic response in canine isolated basilar arteries. In low doses (1 × 10^?8M?1 × 10^?7M) PGI₂ caused a slight but consistent relaxation of resting muscle tone. In low concentrations (1 × 10^?8M?1 × 10^?6M) PGI₂ antagonized muscle contractions caused by serotonin or prostaglandin (PG) F_2α. This relaxant effect with low doses of PGI₂ on the isolated cerebral artery contrasts with findings obtained with other PGs and supports the hypothesis that PGI₂ is a mediator of vasodilatation. However, in 1 × 10^?5M concentrations PGI₂ contracted the arterial muscle and did not antagonize contractions induced by serotonin or PGF_2α.