Adenylate cyclase-mediated vascular responses of rabbit aorta, mesenteric artery and skin microcirculation

The pig is a better model than the rabbit or rat for studying the pathophysiology of human mesenteric arteries

AIMS

Animal models are essential to investigate cardiovascular pathophysiology and pharmacology, but phylogenetic diversity makes it necessary to identify the model with vasculature most similar to that of humans.

METHODS AND RESULTS

In this study, we compared the mesenteric arteries of humans, pigs, rabbits and rats in terms of the i) evolutionary changes in the amino acid sequences of α₁ and β2 adrenoceptors; M₁, M₂, and M₃ muscarinic receptors; and bradykinin (BKR) and thromboxane-prostanoid (TP) receptors, through bioinformatics tools; ii) expression of α₁, β₂, M1, M3 and TP receptors in each tunica, as assessed by immunofluorescence; and iii) reactivity to receptor-dependent and independent contractile agonists and relaxants, by performing organ bath assays. Phylogenetically, pigs showed the highest degree of evolutionary closeness to humans for all receptors, and with the exception of BKR, rabbits presented the greatest evolutionary difference compared to humans, pigs and rats. The expression of the measured receptors in the three vascular tunica in pigs was most similar to that in humans. Using a one-way ANOVA to determine the differences in vascular reactivity, we found that the reactivity of pigs was the most similar to that of humans in terms of sensitivity (pD2) and maximum effect of vascular reactivity (Emax) to KCl, phenylephrine, isoproterenol and carbachol.

CONCLUSIONS

The pig is a better vascular model than the rabbit or rat to extrapolate results to human mesenteric arteries. Comparative vascular studies have implications for understanding the evolutionary history of different species.

TRANSLATIONAL PERSPECTIVE

The presented findings are useful for identifying an animal model with a vasculature that is similar to that of humans. This information is important to extrapolate, with greater precision, the findings in arterial pathophysiology or pharmacology from animal models to the healthy or diseased human being.

Backup Mechanisms Maintain PACAP/VIP-Induced Arterial Relaxations in Pituitary Adenylate Cyclase-Activating Polypeptide-Deficient Mice

BACKGROUND

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a multifunctional neuropeptide in the VIP/secretin/glucagon peptide superfamily. Two active forms, PACAP1-38 and PACAP1-27, act through G protein-coupled receptors, the PAC1 and VPAC1/2 receptors. Effects of PACAP include potent vasomotor activity. Vasomotor activity and organ-specific vasomotor effects of PACAP-deficient mice have not yet been investigated; thus, the assessment of its physiological importance in vasomotor functions is still missing. We hypothesized that backup mechanisms exist to maintain PACAP pathway activity in PACAP knockout (KO) mice. Thus, we investigated the vasomotor effects of exogenous vasoactive intestinal peptide (VIP) and PACAP polypeptides in PACAP wild-type (WT) and PACAP-deficient (KO) male mice.

METHODS

Carotid and femoral arteries were isolated from 8- to 12-week-old male WT and PACAP-KO mice. Vasomotor responses were measured with isometric myography.

RESULTS

In the arteries of WT mice the peptides induced relaxations, which were significantly greater to PACAP1-38 than to PACAP1-27 and VIP. In KO mice, PACAP1-38 did not elicit relaxation, whereas PACAP1-27 and VIP elicited significantly greater relaxation in KO mice than in WT mice. The specific PAC1R and VPAC1R antagonist completely blocked the PACAP-induced relaxations.

CONCLUSION

Our data suggest that in PACAP deficiency, backup mechanisms maintain arterial relaxations to polypeptides, indicating an important physiological role for the PACAP pathway in the regulation of vascular tone.

Pituitary adenylate cyclase-activating polypeptide (PACAP) induces relaxations of peripheral and cerebral arteries, which are differentially impaired by aging

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a well-known neuropeptide, which also has vasomotor effects. However, little is known regarding its age-related and organ-specific vasomotor effects. We hypothesized that the vasomotor effects of PACAP depend on the tissue origin of the vessels and aging substantially modulates its actions. Thus, carotid (CA) and basilar arteries (BA) were isolated from young (2 months old), middle age (12 months old), and old (30 months old) rats. Their vasomotor responses were measured with an isometric myograph (DMT610M) in response to cumulative concentrations of PACAP1-38 (10(-9)-10(-6) M). PACAP1-38 induced (1) significantly greater concentration-dependent relaxations in CA compared to that of BA of young, middle age, and old rats; (2) relaxations of CA significantly decreased, whereas they did not change substantially in BA, as a function of age; (3) sodium nitroprusside (SNP)-induced relaxation did not change after PACAP1-38 administration in any conditions; and (4) inhibition of PAC1 receptors by selective PAC1 receptor blocker (PACAP6-38) completely diminished the responses to PACAP in all age groups of BA and CA. In conclusion, these findings suggest that PACAP1-38 has greater vasomotor effect in CA than that in BA, whereas aging has less effect on PACAP-induced relaxation of cerebral arteries and BA than that in peripheral arteries and CA suggesting that the relaxation to PACAP is maintained in cerebral arteries even in old age.

Pituitary adenylate cyclase activating polypeptide (PACAP) dilates cerebellar arteries through activation of large-conductance Ca(2+)-activated (BK) and ATP-sensitive (K ATP) K (+) channels

Pituitary adenylate cyclase activating polypeptide (PACAP) is a potent vasodilator of numerous vascular beds, including cerebral arteries. Although PACAP-induced cerebral artery dilation is suggested to be cyclic AMP (cAMP)-dependent, the downstream intracellular signaling pathways are still not fully understood. In this study, we examined the role of smooth muscle K(+) channels and hypothesized that PACAP-mediated increases in cAMP levels and protein kinase A (PKA) activity result in the coordinate activation of ATP-sensitive K(+) (KATP) and large-conductance Ca(2+)-activated K(+) (BK) channels for cerebral artery dilation. Using patch-clamp electrophysiology, we observed that PACAP enhanced whole-cell KATP channel activity and transient BK channel currents in freshly isolated rat cerebellar artery myocytes. The increased frequency of transient BK currents following PACAP treatment is indicative of increased intracellular Ca(2+) release events termed Ca(2+) sparks. Consistent with the electrophysiology data, the PACAP-induced vasodilations of cannulated cerebellar artery preparations were attenuated by approximately 50 % in the presence of glibenclamide (a KATP channel blocker) or paxilline (a BK channel blocker). Further, in the presence of both blockers, PACAP failed to cause vasodilation. In conclusion, our results indicate that PACAP causes cerebellar artery dilation through two mechanisms: (1) KATP channel activation and (2) enhanced BK channel activity, likely through increased Ca(2+) spark frequency.

Evidence for the involvement of VPAC1 and VPAC2 receptors in pressure-induced vasodilatation in rodents

A transient increase in skin blood flow in response to an innocuous local pressure application, defined as pressure-induced vasodilatation (PIV), delays the occurrence of ischaemia, suggesting a protective feature against applied pressure. The PIV response depends on capsaicin-sensitive nerve fibres and calcitonin gene-related peptide (CGRP) has been shown to be involved. In these fibres, CGRP coexists with pituitary adenylate cyclase-activating polypeptide (PACAP). Three distinct receptors mediate the biological effects of PACAP: VPAC1 and VPAC2 receptors binding with the same affinity for PACAP and vasoactive intestinal peptide and PAC1 receptors showing high selectivity for PACAP. Because the receptors are widely expressed in the nervous system and in the skin, we hypothesized that at least one of them is involved in PIV development. To verify this hypothesis, we used [D-p-Cl-Phe(6),Leu(17)]-VIP (nonspecific antagonist of VPAC1/VPAC2 receptors), PG 97-269 (antagonist of VPAC1 receptors), PACAP(6-38) (antagonist of VPAC2/PAC1 receptors) and Max.d.4 (antagonist of PAC1 receptors) in anaesthetized rodents. The blockade of VPAC1/VPAC2, VPAC1 or VPAC2/PAC1 receptors eliminated the PIV response, whereas PAC1 blockade had no effect, demonstrating an involvement of VPAC1/VPAC2 receptors in PIV development. Moreover, endothelium-independent and -dependent vasodilator responses were unchanged by the VPAC1/VPAC2 antagonist. Thus, the absence of a PIV response following VPAC1/VPAC2 blockade cannot be explained by any dysfunction of the vascular smooth muscle or endothelial vasodilator capacity. The involvement of VPAC1/VPAC2 receptors in the development of PIV seems to imply a series relationship in which each receptor type (CGRP, VPAC1, VPAC2) is necessary for the full transmission of the response.

Characterization of beta-adrenoceptor subtypes in the ferret urinary bladder in vitro and in vivo

In the present study, the beta-adrenoceptor subtypes distributed in the detrusor of the ferret were investigated in functional experiments in vitro and in vivo using a variety of beta-adrenoceptor agonists and antagonists. All the beta-adrenoceptor agonists tested relaxed the isolated detrusor strip, the rank order of potency being (+/-)-(R*, R*)-[4-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino]propyl]phenoxy]- acetic acid sodium (BRL 37344A)>(+/-)-4-(3-t-butylamino-2-hydroxypropoxy) benzimidazol-2-one (CGP-12177A), isoprenaline and (R, R)-5-[2-[[2-(3-chlorophenyl)-2-hydroxyethylamino]propyl]-1, 3-benzodioxole-2,2-dicarboxylate (CL 316,243)>dobutamine and procaterol. In antagonist experiment, 3-(2-allylphenoxy)-1-[(1S)-1,2, 3,4-tetrahydro-naphth-1-ylamino]-(2S)-2-propanol hydrochloride (SR 58894A), but neither 2-hydroxy-5(2-((2-hydroxy-3-(4-((1-methyl-4-trifluoromethyl)1H-imidaz ole-2-yl)-phenoxy)propyl)amino)ethoxy)-benzamide monomethane sulphonate (CGP-20712A) nor erythro-(+/-)-1-(7-methylindan-4-yloxy)-3-isopropylaminob utan-2-ol hydrochloride (ICI-118,551), caused a rightward shift of the concentration-relaxation curve for isoprenaline. In in vivo experiments, isoprenaline and CL 316,243 each reduced bladder pressure in a dose-dependent manner. CL 316,243 was the only drug that did not produce any significant influences on blood pressure and heart rate at doses that reduced bladder pressure. The present functional study provides the first evidence that relaxation of the ferret detrusor by beta-adrenoceptor activation is mediated mainly via the beta(3)-adrenoceptor, as in the human detrusor.

Hemodynamic actions of systemically injected pituitary adenylate cyclase activating polypeptide-27 in the rat

The aims of this study were (1) to characterize the hemodynamic mechanisms underlying the hypotensive effects of pituitary adenylate cyclase activating polypeptide-27 (PACAP-27 0.1-2.0 nmol/kg, i.v.) in pentobarbital-anesthetized rats, and (2) to determine the roles of the autonomic nervous system, adrenal catecholamines and endothelium-derived nitric oxide (NO) in the expression of PACAP-27-mediated effects on hemodynamic function. PACAP-27 produced dose-dependent decreases in mean arterial blood pressure and hindquarter and mesenteric vascular resistances in saline-treated rats. PACAP-27 also produced pronounced falls in mean arterial blood pressure in rats treated with the ganglion blocker, chlorisondamine (5 mg/kg, i.v.). The hypotensive and vasodilator actions of PACAP-27 were not attenuated by the beta-adrenoceptor antagonist, propranolol (1 mg/kg, i.v.), or the NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME 50 micromol/kg, i.v.). PACAP-27 produced dose-dependent increases in heart rate whereas the hypotensive response produced by the nitrovasodilator, sodium nitroprusside (10 microg/kg, i.v.), was associated with a minimal tachycardia. The PACAP-27-induced tachycardia was unaffected by chlorisondamine, but was virtually abolished by propranolol. These results suggest that the vasodilator effects of PACAP-27 are due to actions in the microcirculation rather than to the release of adrenal catecholamines and that this vasodilation may not involve the release of endothelium-derived NO. These results also suggest that PACAP-27 produces tachycardia by directly releasing norepinephrine from cardiac sympathetic nerve terminals rather than by direct or baroreceptor reflex-mediated increases in sympathetic nerve activity.

Differences in responses to norepinephrine and adenosine triphosphate in isolated, perfused mesenteric vascular beds between normotensive and spontaneously hypertensive rats

The responses to norepinephrine and adenosine 5' triphosphate (ATP) of isolated, perfused mesenteric vascular beds were compared between spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. Norepinephrine (0.01-100 nmol) dose-dependently increased perfusion pressure in the intact bed and the arteries, but not in the veins. The maximal responses in SHRs were larger than those in WKY rats. ATP (0.1-3,000 nmol) increased perfusion pressure in all preparations. The responses of the intact bed and the veins were larger in SHRs, whereas there was no strain difference in the arteries. Indomethacin (5 x 10(-6) M) enlarged the norepinephrine responses of both strains only in the intact beds and did not affect the ATP responses, except the veins in SHRs, where it was reduced. N(G)-nitro-L-arginine methyl ester (5 x 10(-6) M), in combination with indomethacin, potentiated the responses, except the arterial response to high doses of norepinephrine in SHRs, which was not affected. Endothelium denudation in the arteries produced similar changes to those after the combined treatment. UK14,304-induced and ADPbetaS-induced decreases in perfusion pressure at increased tone were similar between the strains. Thus neither the vasodilation induced by the stimulation of alpha2-adrenoceptors nor of P2y receptors seems to affect the response to norepinephrine or to ATP, respectively. These results demonstrate that the intact mesenteric vascular bed of SHRs shows potentiated responses not only to norepinephrine, but also to ATP, as compared with WKY rats, and that the critical regions for determining the strain differences for norepinephrine are overall arteries, and that for ATP are the vessels downstream from arterioles. In the intact beds, neither regulation by endogenous prostanoids nor that by endothelium-derived relaxing factors (EDRFs) is implicated in the strain difference. However, these two types of regulation differ markedly between different kinds of vessels.

Prostaglandin E1 potentiation of the spontaneous phasic contraction of rat isolated portal vein by a cyclopiazonic acid-sensitive mechanism

1. The effect of prostaglandin E1 (PGE1) on the spontaneous phasic contraction of the rat isolated portal vein was studied. 2. The isolated portal vein exhibited spontaneous phasic contractions. Removal of Ca2+ from Krebs-Ringer solution or application of nifedipine abolished the spontaneous contraction, indicating that the contraction depends exclusively on Ca2+ influx through L-type Ca2+ channels. On the other hand, cyclopiazonic acid (CPA), a specific inhibitor of Ca(2+)-ATPase of sarcoplasmic reticulum (SR) increased the amplitude of the contractions, suggesting that the SR regulates the spontaneous contractions negatively by sequestration of Ca2+ entering through L-type Ca2+ channels and buffering the rise in cytosolic Ca2+. 3. PGE1 increased the amplitude of the spontaneous contraction in a concentration-dependent manner without affecting the resting tension. The effect was completely abolished by nifedipine. Bay K 8644 and phenylephrine (PE) also increased the amplitude of the contraction in a concentration-dependent manner. PGE1 at a concentration of 1 microM. Bay K 8644 at 100 nM and PE at 30 nM doubled the amplitude, respectively. 4. Pretreatment with 1 microM CPA abolished the effect of PGE1, but the effects of Bay K 8644 and PE were not inhibited by pretreatment with CPA. In contrast, 10 microM ryanodine attenuated the effect of PE without affecting the contractile effect of PGE1. 5. When the SR was depleted of Ca2+ by repeated applications of caffeine in a nominally Ca(2+)-free Krebs-Ringer solution, it took about 120 s to restore the spontaneous contraction after addition of Ca2+ into the solution. In CPA-treated veins, the time taken to restore the contraction was shortened significantly. Pretreatment with 1 microM PGE1 shortened the time to the same extent as pretreatment with CPA did. 6. These results suggest that PGE1 increases the amplitude of the spontaneous phasic contraction by a different mechanism from those by which PE and Bay K 8644 increase it. Inhibition of Ca(2+)-ATPase of the SR might be involved in the vasoactive effect of PGE1.

Arteriolar dilation to endotoxin is increased in copper-deficient rats

We have previously reported that there is an altered response to mast cell-mediated inflammation in copper-deficient rats. In the current study we determined the microvascular reactivity to inflammatory stimuli with lipopolysacccharide (LPS) during dietary copper restriction. Male Sprague-Dawley rats were fed purified diets which were either copper-adequate (CuA, 6 micrograms Cu/g) or copper-deficient (CuD, 0.4 micrograms Cu/g) for 4 weeks. Rats were anesthetized and the cremaster muscle was prepared for in vivo television microscopy. Arteriolar diameters were measured and then 2.5 mg/kg LPS was injected i.p. In separate groups, animals were pretreated with the NO-synthase inhibitor L-NAME (2 x 10(-4) M), the cyclooxygenase inhibitor ibuprofen (9.6 x 10(-5) M) or the histamine receptor antagonist diphenhydramine (DPH, 10(-6) M). LPS caused arteriolar dilation in both dietary groups with the response being significantly greater in the CuD group. Ibuprofen and DPH but not L-NAME, each significantly reduced but did not block the dilation in the CuD group. Ibuprofen and DPH together blocked the dilation. These results suggest that dietary copper deficiency increases arteriolar dilation to LPS. The mechanism appears to involve a greater response to arachidonic acid metabolites and histamine but not NO.

Effects of beraprost sodium and prostaglandin E1 on skin blood flow in diabetic rats and normal dogs

1. The present study was undertaken to compare the effects of beraprost sodium (BPS), a stable prostaglandin I2 analog, with prostaglandin E1 (PGE1) on the skin blood flow in anesthesized diabetic rats and anesthetized dogs. 2. In anesthetized diabetic rats, BPS, but not PGE1, caused a dose-dependent and significant increase in femoral skin blood flow. 3. In anesthetized dogs, PGE1 significantly increased femoral arterial blood flow than BPS. However, BPS significantly increased in-step skin blood flow than PGE1. 4. The above data suggest that BPS may be clinically useful for the treatment of circulatory insufficiency of lower extremities in diabetic patients.

cGMP mediates the vascular and platelet actions of nitric oxide: confirmation using an inhibitor of the soluble guanylyl cyclase

The L-arginine:nitric oxide (NO) pathway is believed to exert many of its physiological effects via stimulation of the soluble guanylyl cyclase (SGC); however, the lack of a selective inhibitor of this enzyme has prevented conclusive demonstration of this mechanism of action. We have found that the compound 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ) inhibits the elevation of cGMP induced by the NO donor S-nitroso-DL-penicillamine in human platelets and rat vascular smooth muscle (IC50 = 10-60 nM and <10 nM, respectively) and that this is accompanied by prevention of the platelet inhibitory and vasodilator actions of NO donors. ODQ also inhibited the antiaggregatory action of NO generated by the platelets but did not affect the action of prostacyclin or that of a cGMP mimetic. In addition, ODQ inhibited the vasodilator actions of endogenously released NO and of NO generated after induction of NO synthase in vascular preparations. It did not, however, affect the increase in vascular smooth muscle cGMP or the dilatation induced by atrial natriuretic factor. ODQ had no effect on NO synthase activity, nor did it react with NO. It did, however, potently (IC50 approximately 10 nM) inhibit the activity of the SGC in cytosol obtained from crude extract of rat aortic smooth muscle. Thus ODQ prevents the actions of NO on platelets and vascular smooth muscle through its potent inhibitory effect on the SGC.

Alpha 2-adrenoceptor mediated inhibition of forskolin-stimulated cyclic AMP accumulation in isolated porcine palmar lateral veins

The aim of this study was to use a 3H-adenine pre-labelling technique to characterise the effect of alpha 2-adrenoceptor activation on forskolin-stimulated cyclic AMP accumulation in the isolated porcine palmar lateral vein. Forskolin (10(-7)-10(-4) M) stimulated 3H-cyclic AMP accumulation in the isolated porcine palmar lateral vein in a biphasic and concentration-dependent manner. In the absence of the cyclic AMP-selective phosphodiesterase inhibitor rolipram, forskolin stimulated 3H-cyclic AMP accumulation approximately 7-8 fold. The response reached a peak after 5 min. In the presence of rolipram (10(-5) M), basal 3H-cyclic AMP levels were approximately 70% higher than in its absence (basal: 1823 +/- 57 dpm; rolipram: 3088 +/- 229, n = 3) and forskolin (3 x 10(-5) M) stimulated 3H-cyclic AMP accumulation approximately 8 fold. The latter response reached a plateau 10 min after the addition of forskolin. In all subsequent studies, the tissues were incubated with forskolin (3 x 10(-5) M) for 5 min in the absence of rolipram. Noradrenaline (NA; 10(-9)-10(-4) M) and UK14304 (10(-9)-10(-4) M) inhibited forskolin-stimulated 3H-cyclic AMP accumulation in a concentration-dependent manner with mean pIC50 values of 7.61 +/- 0.37 (n = 4) and 7.76 +/- 0.23 (n = 5), respectively. With either NA or UK14304, the maximal inhibition of the forskolin response obtained was approximately 75%. Neither NA (10(-4) M) nor UK14304 (10(-4) M) altered basal 3H-cyclic AMP levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissimilarity between prostaglandin E1 and nitric oxide donors as potentiators of plasma exudation in the rabbit skin in vivo

The ability of prostaglandin E1 (PGE1) and nitric oxide (NO) donor compounds such as sodium nitroprusside (SNP), glyceryl trinitrate (GTN), and 3-morpholino-sydnonimine (SIN-1) to modulate the histamine- and bradykinin-induced increase in microvascular permeability have been investigated in rabbit skin. The effect of the NO synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) on the plasma exudation induced by histamine and bradykinin was also studied. Local edema formation was evaluated using [125I]human serum albumin. New Zealand white rabbits received an intravenous injection of [125I]human albumin followed immediately by the intradermal injection of edematogenic agents into the shaved dorsolateral skin. PGE1 (0.1 nmol/site) significantly potentiated both histamine- and bradykinin-induced edema. In contrast, SNP (0.4-400 nmol/site), SIN-1 (0.4-400 nmol/site), and GTN (0.4-40 nmol/site) did not affect the edematogenic response induced by either histamine or bradykinin. GTN (0.4-40 nmol/site) also had no effect on the increase in plasma exudation induced by histamine and bradykinin in the presence of PGE1. L-NAME (50-400 nmol/site, but not its enantiomer D-NAME, dose-dependently reduced the edema formation induced by a combination of either histamine or bradykinin with PGE1. This inhibition was significantly reversed by SNP (4-400 nmol/site) and by high doses (2.5 mumol/site) of L-arginine (but not by D-arginine). Our results thus demonstrate that PGE1, but not nitrovasodilators, can actually potentiate histamine- and bradykinin-induced edema in rabbit skin.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide-based possibilities for pharmacotherapy

The goal of nitric oxide (NO) based pharmacotherapy is to reach proper homeostasis of NO metabolism in the target tissue where endogenous production of NO is either too weak or excessively increased. In addition to the classic NO-based therapy of cardiovascular conditions with nitrates, a variety of new therapeutic possibilities have emerged including sexual disorders, gastrointestinal system, immunology, tumour growth regulation and respiratory disorders. NO levels of target tissues can be affected directly by NO donors, or indirectly by increasing the level of L-arginine, a substrate of nitric oxide synthase (NOS). While increased production of NO by induceable NO (iNOS) by, for example, cytokines does not at present seem therapeutically meaningful, increased NO production by constitutive NOS (cNOS) may be involved in the beneficial effects of ACE inhibitors or oestrogens. NO production may be pharmacologically decreased by inhibition of expression of iNOS by glucocorticoids while both cNOS and iNOS derived NO production is inhibited by administration of false substrates, for example L-NAME. Additionally, the respiratory system and related vessels can be reached directly and more selectively by inhalation of pure NO gas. Possible problems in administering NO and perhaps some NO-donors include the toxic nature of the compound itself whereby vital enzyme systems may be inhibited and tissue damaging radicals formed. Future prospects of NO-based pharmacotherapy may feature selective ligands to different NOS isoforms and tissue selective donors that release NO in a controlled fashion.

Potentiation by forskolin of both SNP- and ANP-stimulated cyclic GMP accumulation in porcine isolated palmar lateral vein

1. The aim of this study was to examine the effect of modulation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels (by using forskolin, a direct activator of adenylyl cyclase, or rolipram, a cyclic AMP selective phosphodiesterase inhibitor) on basal and stimulated guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels in the porcine isolated palmer lateral vein by use of a [3H]-guanine prelabelling technique. 2. Sodium nitroprusside (SNP; 10(-5) - 10(-3) M) and atrial natriuretic peptide (ANP; 10(-8) - 10(-6) M), produced concentration-dependent increases in [3H]-cyclic GMP levels via stimulation of soluble and particulate guanylyl cyclase respectively. The SNP-stimulated [3H]-cyclic GMP response peaked after 5 min in the presence and absence of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). 3. In the absence of IBMX, forskolin (3 x 10(-5) M) significantly increased [3H]-cyclic GMP levels to 118.5 +/- 8.7% of basal values (P < 0.05, n = 8), and significantly increased both the SNP- and ANP-stimulated [3H]-cyclic GMP accumulation at all concentrations of SNP and ANP used. For example, effects at the maximal SNP (10(-3) M) and ANP (10(-6) M) concentrations were: SNP: 154.7 +/- 15.4% of basal; SNP+forskolin: 191.3 +/- 14.8% of basal (P < 0.05, n = 4); ANP: 161.4 +/- 17.4% of basal; ANP+forskolin: 220.0 +/- 20.0% of basal (P < 0.05, n = 4). 4. The cyclic AMP-selective phosphodiesterase inhibitor, rolipram (10-5 M), had no effect on basal or SNP-stimulated [3H]-cyclic GMP levels; however, the combination of forskolin and rolipram produced an increase in the basal (158.7 +/- 27.1% of basal) and SNP-stimulated [3H]-cyclic GMP accumulation(SNP (10-3 M): 165.3 +/- 8.7% of basal; SNP + forskolin + rolipram: 510.7 +/- 64.8% of basal; P<0.05,n = 5), greater than either forskolin or rolipram alone. The phosphodiesterase inhibitor, IBMX (10-3 M)significantly raised [3H]-cyclic GMP levels, and forskolin (3 x 10- M) in the presence of IBMX had no significant effect on either basal or SNP-stimulated [3H]-cyclic GMP levels (e.g. in the presence of IBMX: SNP (10-3 M): 660 +/- 90% of basal; SNP + forskolin: 790 +/- 86% of basal, n = 3).5. The data indicate the presence of both soluble and particulate guanylyl cyclase in the porcine isolated palmar lateral vein. The ability of forskolin to potentiate SNP- and ANP-stimulated [3H]-cyclic GMP accumulation may suggest a cyclic AMP-cyclic GMP interaction at the level of the phosphodiesterases.Further, the ability of cyclic AMP to influence cyclic GMP levels may indicate that the two nucleotides, as well as having independent mechanisms to induce smooth muscle relaxation, could produce vasodilatation via a common mechanism.