Effects of a water-soluble forskolin derivative (NKH477) and a membrane-permeable cyclic AMP analogue on noradrenaline-induced Ca2+ mobilization in smooth muscle of rabbit mesenteric artery

NO-induced vasodilation correlates directly with BP in smooth muscle-Na/Ca exchanger-1-engineered mice: elevated BP does not attenuate endothelial function

Arterial smooth muscle Na+/Ca2+ exchanger-1 (SM-NCX1) promotes vasoconstriction or vasodilation by mediating, respectively, Ca2+ influx or efflux. In vivo, SM-NCX1 mediates net Ca2+ influx to help maintain myogenic tone (MT) and neuronally activated constriction. SM-NCX1-TG (overexpressing transgenic) mice have increased MT and mean blood pressure (MBP; +13.5 mmHg); SM-NCX1-KO (knockout) mice have reduced MT and MBP (-11.1 mmHg). Endothelium-dependent vasodilation (EDV) is often impaired in hypertension. We tested whether genetically engineered SM-NCX1 expression and consequent BP changes similarly alter EDV. Isolated, pressurized mesenteric resistance arteries with MT from SM-NCX1-TG and conditional SM-NCX1-KO mice, and femoral arteries in vivo from TG mice were studied. Acetylcholine (ACh)-dilated TG arteries with MT slightly more than control or KO arteries, implying that SM-NCX1 overexpression does not impair EDV. In preconstricted KO, but not TG mouse arteries, however, ACh- and bradykinin-triggered vasodilation was markedly attenuated. To circumvent the endothelium, phenylephrine-constricted resistance arteries were tested with Na-nitroprusside [SNP; nitric oxide (NO) donor] and cGMP. This endothelium-independent vasodilation was augmented in TG but attenuated in KO arteries that lack NCX1-mediated Ca2+ clearance. Baseline cytosolic Ca2+ ([Ca2+]cyt) was elevated in TG femoral arteries in vivo, supporting the high BP; furthermore, SNP-triggered [Ca2+]cyt decline and vasodilation were augmented as NO and cGMP promote myocyte polarization thereby enhancing NCX1-mediated Ca2+ efflux. The TG mouse data indicate that BP elevation does not attenuate endothelium-dependent vasodilation. Thus, in essential hypertension and many models the endothelial impairment that supports the hypertension apparently is not triggered by BP elevation but by extravascular mechanisms.NEW & NOTEWORTHY Endothelium-dependent, ACh-induced vasodilation (EDV) is attenuated, and arterial myocyte Na+/Ca2+ exchangers (NCX1) are upregulated in many forms of hypertension. Surprisingly, mildly hypertensive smooth muscle-specific (SM)-NCX1 transgenic mice exhibited modestly enhanced EDV and augmented endothelium-independent vasodilation (EIV). Conversely, mildly hypotensive SM-NCX1-knockout mice had greatly attenuated EIV. These adaptations help compensate for NCX1 expression-induced alterations in cytosolic Ca2+ and blood pressure (BP) and belie the view that elevated BP, itself, causes the endothelial dysregulation in hypertension.

Sustained signalling by PTH modulates IP3 accumulation and IP3 receptors through cyclic AMP junctions

Parathyroid hormone (PTH) stimulates adenylyl cyclase through type 1 PTH receptors (PTH₁R) and potentiates the Ca²⁺ signals evoked by carbachol, which stimulates formation of inositol 1,4,5-trisphosphate (IP₃). We confirmed that in HEK cells expressing PTH₁R, acute stimulation with PTH(1-34) potentiated carbachol-evoked Ca²⁺ release. This was mediated by locally delivered cyclic AMP (cAMP), but unaffected by inhibition of protein kinase A (PKA), exchange proteins activated by cAMP, cAMP phosphodiesterases (PDEs) or substantial inhibition of adenylyl cyclase. Sustained stimulation with PTH(1-34) causes internalization of PTH₁R–adenylyl cyclase signalling complexes, but the consequences for delivery of cAMP to IP₃R within cAMP signalling junctions are unknown. Here, we show that sustained stimulation with PTH(1-34) or with PTH analogues that do not evoke receptor internalization reduced the potentiated Ca²⁺ signals and attenuated carbachol-evoked increases in cytosolic IP₃. Similar results were obtained after sustained stimulation with NKH477 to directly activate adenylyl cyclase, or with the membrane-permeant analogue of cAMP, 8-Br-cAMP. These responses were independent of PKA and unaffected by substantial inhibition of adenylyl cyclase. During prolonged stimulation with PTH(1-34), hyperactive cAMP signalling junctions, within which cAMP is delivered directly and at saturating concentrations to its targets, mediate sensitization of IP₃R and a more slowly developing inhibition of IP₃ accumulation.

Colforsin-induced vasodilation in chronic hypoxic pulmonary hypertension in rats

PURPOSE

Colforsin, a water-soluble forskolin derivative, directly activates adenylate cyclase and thereby increases the 3',5'-cyclic adenosine monophosphate (cAMP) level in vascular smooth muscle cells. In this study, we investigated the vasodilatory action of colforsin on structurally remodeled pulmonary arteries from rats with pulmonary hypertension (PH).

METHODS

A total of 32 rats were subjected to hypobaric hypoxia (380 mmHg, 10% oxygen) for 10 days to induce chronic hypoxic PH, while 39 rats were kept in room air. Changes in isometric force were recorded in endothelium-intact (+E) and -denuded (-E) pulmonary arteries from the PH and control (non-PH) rats.

RESULTS

Colforsin-induced vasodilation was impaired in both +E and -E arteries from PH rats compared with their respective controls. Endothelial removal did not influence colforsin-induced vasodilation in the arteries from control rats, but attenuated it in arteries from PH rats. The inhibition of nitric oxide (NO) synthase did not influence colforsin-induced vasodilation in +E arteries from controls, but attenuated it in +E arteries from PH rats, shifting its concentration-response curve closer to that of -E arteries from PH rats. Vasodilation induced by 8-bromo-cAMP (a cell-permeable cAMP analog) was also impaired in -E arteries from PH rats, but not in +E arteries from PH rats, compared with their respective controls.

CONCLUSIONS

cAMP-mediated vasodilatory responses without beta-adrenergic receptor activation are impaired in structurally remodeled pulmonary arteries from PH rats. In these arteries, endothelial cells presumably play a compensatory role against the impaired cAMP-mediated vasodilatory response by releasing NO (and thereby attenuating the impairment). The results suggest that colforsin could be effective in the treatment of PH.

Effects of formoterol and BRL 37344 on human umbilical arteries in vitro in normotensive and pre-eclamptic pregnancy

Alterations in vascular responses to beta-adrenoceptor agonists in normotensive pregnancy and pre-eclampsia are not fully understood. Thus, we studied changes in vasodilator responses to beta(2)-adrenoceptor agonist formoterol and beta(3)-adrenoceptor agonist BRL 37344 on umbilical arteries isolated from normotensive (n=12) and pre-eclamptic (n=12) pregnant women. Changes in the relaxant effect of formoterol and BRL 37344 were investigated by measuring isometric tensions in endothelium-denuded strips of umbilical arteries in the presence or absence of metoprolol, ICI 118.551 and SR 59230A (beta(1), beta(2), beta(3)-adrenoceptor antagonists, respectively, 10(-6) mol/L). Effects of formoterol and BRL 37344 on cAMP levels of umbilical arteries were evaluated by radioimmunoassay kits. Formoterol (10(-10)-10(-4) mol/L) and BRL 37344 (10(-10)-10(-4) mol/L) caused concentration-dependent relaxation of the contraction induced by phenylephrine (10(-5) mol/L) in umbilical artery strips isolated from both groups. E(max) values of formoterol and BRL 37344 (for normotensive pregnant women: 87.33+/-0.87 and 53.25+/-1.17 vs. for pre-eclampsia: 73.68+/-1.58 and 43.64+/-1.19, n=12, P>0.05, respectively) were significantly smaller in strips from pre-eclamptic women (P<0.05), with no significant change in pD(2) values. E(max) values of formoterol were significantly higher than those of BRL 37344 in both tissue (P<0.05). ICI 118.551 and SR 59230A, but not metoprolol, antagonized the relaxant effects of formoterol and of BRL 37344 on umbilical artery strips isolated from normotensive and pre-eclamptic pregnant women. Formoterol and BRL 37344 increased cAMP levels in both groups, but less significant in pre-eclamptic strips (P<0.05). These results suggest that the relaxation caused in human umbilical arteries by formoterol and BRL 37344 is mediated by a mixed population of beta(2)- and beta(3)-adrenoceptor subtypes, with contribution of cAMP. Umbilical arteries from subjects with pre-eclampsia showed a weaker beta(2)- and beta(3)-receptor-mediated relaxation to formoterol and BRL 37344, suggesting that the reduced action of formoterol and BRL 37344 may be partly due to a decreased effect of cAMP.

Intraarterial injection of colforsin daropate hydrochloride for the treatment of vasospasm after aneurysmal subarachnoid hemorrhage: preliminary report of two cases

We describe two patients with symptomatic vasospasms after aneurysmal subarachnoid hemorrhage who were successfully treated with intraarterial injection of colforsin daropate hydrochloride (HCl). Colforsin daropate HCl is capable of directly stimulating adenylate cyclase, which in turn causes vasorelaxation via elevated intracellular concentrations of cyclic adenosine monophosphate. We suggest that colforsin daropate HCl might be a useful therapeutic tool in treating cerebral vasospasm.

Influence of colforsin daropate hydrochloride on internal mammary artery grafts

The effect of colforsin daropate hydrochloride (colforsin), a water-soluble forskolin derivative, on blood flow in internal mammary artery (IMA) grafts was evaluated in a prospective randomized study of 26 patients undergoing coronary artery bypass grafting. Patients were randomized to receive either colforsin treatment (colforsin; n=14) or no colforsin treatment (control; n=14). Administration of colforsin (0.5mg x kg(-1) min(-1)) was started after induction of anesthesia and was continued for 6 h. IMA blood flow and hemodynamic measurements were assessed perioperatively. During cardiopoulmonary bypass (CPB), perfusion flow was adjusted to 2.5 L/m2 and IMA free blood flow was measured. IMA blood flow was also measured 1 h after CPB by an ultrasonic flow meter. Systemic vascular resistance was significantly lower in the colforsin group during and after CPB. IMA blood flow was significantly greater in the colforsin group than in the control group during (44 +/- 2 vs 33 +/- 3 ml min-1 x m(-2), p=0.02) and after CPB (38 +/- 6 vs 20 +/- 3ml x min(-1) m(-2), p=0.01). IMA blood flow 1 h after CPB correlated inversely with concurrent systemic vascular resistance (r=-0.61, p=0.001). Intraoperative administration of colforsin daropate hydrochloride caused potent vasodilation, resulting in an increase in IMA blood flow. The results indicate that the regimen can be used perioperatively in patients undergoing coronary artery bypass grafting.

Platelet-derived growth factor causes endothelium-independent relaxation of rabbit mesenteric artery via the release of a prostanoid

Recent evidence has indicated that the mitogen platelet-derived growth factor (PDGF) can act acutely to regulate arterial tone. In this study we demonstrate that the BB isoform of PDGF (PDGF-BB) can cause endothelium-independent relaxation of rabbit isolated mesenteric arteries. In endothelium-denuded arteries, PDGF-BB (40 pM - 8.0 nM) caused concentration-dependent relaxation of noradrenaline-induced tone. The relaxation to PDGF-BB was abolished by a cyclo-oxygenase inhibitor, indomethacin (10 microM) and by the PDGF receptor-associated tyrosine kinase inhibitor, tyrphostin AG1295 (50 microM), but not by N:(G)-monomethyl-L-arginine (L-NMMA, 200 microM), an inhibitor of nitric oxide (NO) synthase. PDGF-BB (4.0 nM) significantly increased the release of prostacyclin (PGI(2)) in endothelium-denuded arteries. Exogenously applied iloprost (1 microM), a stable analogue of PGI(2), relaxed the endothelium-denuded artery. PDGF-BB (4.0 nM) significantly increased the cyclic AMP content. In the absence of Ca(2+), PDGF-BB (4.0 nM) failed to relax the artery, and the release of PGI(2) was almost completely suppressed. In addition, the release of PGI(2) by PDGF-BB (4.0 nM) was significantly reduced in the presence of endothelium. The effect of endothelium was eliminated by L-NMMA (200 microM) and PGI(2) release increased. These data indicate that in rabbit mesenteric arteries, PDGF-BB can evoke endothelium-independent relaxation by stimulating the synthesis of PGI(2). The PDGF-BB-induced prostaglandin synthesis is dependent on both Ca(2+) and tyrosine phosphorylation of the PDGF receptor, and is attenuated by endothelium-derived NO.

Characterization of changes in mechanical responses to histamine in omental resistance arteries in pre-eclampsia

Changes in the effect of histamine on the smooth muscle of resistance arteries in pre-eclampsia were investigated by measuring isometric contractions in endothelium-denuded strips of omental resistance arteries from pre-eclamptic and normotensive pregnant women (pregnancy-term matched). Histamine (0.03 -1 microM) caused concentration-dependent relaxation of the contraction induced by 9, 11-epithio-11,12-methano-thromboxane A(2) (STA(2)) in strips from both groups. Sensitivity (for pre-eclampsia: pD(2)=6.66+/-0.04, n=5 and for normotensive pregnant women: pD(2)=7.07+/-0.03, n=10, P<0.001) was lower and the maximum response (90.6+/-0.6% vs 95.5+/-1.1%, P<0.05) was smaller in strips from pre-eclamptic women. Although 8-bromoadenosine-3', 5'-cyclic monophosphorothioate (Sp-isomer: Sp-8-Br-cAMPS, 0.1 - 0.3 mM), a phosphodiesterase (PDE)-resistant activator of adenosine-3',5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase, concentration-dependently attenuated the contraction induced by STA(2) in strips from both groups, the sensitivity (for pre-eclampsia: pD(2)=3.68+/-0.04, n=5 and for normotensive pregnant women: 3.94+/-0.09, n=7, P:=0.02) was lower and the maximum response (64.2+/-2.4% vs 74.9+/-4.4%, P:<0.05) was smaller in pre-eclampsia. In beta-escin-skinned strips, the pD(2) value for the contraction-inducing effect of Ca(2+) did not differ significantly between the two groups (for pre-eclampsia, n=6; for normotensive pregnant women, n=6). Thus, omental resistance arteries from human subjects with pre-eclampsia showed (i) a weaker H(2)-receptor-mediated relaxation to histamine and (ii) a weaker cyclic AMP-analogue-induced relaxation, suggesting that the reduced action of histamine may be partly due to a decreased effect of cyclic AMP.

Possible mechanisms underlying the vasodilatation induced by olprinone, a phosphodiesterase III inhibitor, in rabbit coronary artery

The possible mechanisms underlying the vasodilatation induced by olprinone, a phosphodiesterase type III inhibitor, were investigated in smooth muscle of the rabbit coronary artery. Isometric force and membrane potential were measured simultaneously using endothelium-denuded smooth muscle strips. Acetylcholine (ACh, 3 microM) produced a contraction with a membrane depolarization (15. 2+/-1.1 mV). In a solution containing 5.9 mM K(+), olprinone (100 microM) hyperpolarized the resting membrane and (i) caused the absolute membrane potential level reached with ACh to be more negative (but did not reduce the delta membrane potential seen with ACh, 15.2+/-1.8 mV) and (ii) attenuated the ACh-induced contraction. In a solution containing 30 mM K(+), these effects were not seen with olprinone. Glibenclamide (10 microM) blocked the olprinone-induced membrane hyperpolarization. 4-AP (0.1 mM) significantly attenuated the olprinone-induced resting membrane hyperpolarization but TEA (1 mM) had no such effect. Glibenclamide (10 +microM), TEA (1 mM) and 4-AP (0.1 mM), given separately, all failed to modify the inhibitory actions of olprinone on (i) the absolute membrane potential level seen with ACh and (ii) the ACh-induced contraction. It is suggested that olprinone inhibits the ACh-induced contraction through an effect on the absolute level of membrane potential achieved with ACh in smooth muscle of the rabbit coronary artery. It is also suggested that glibenclamide-sensitive, ATP-sensitive K(+) channels do not play an important role in the olprinone-induced inhibition of the ACh-induced contraction.

Age-related endothelium-dependent vascular relaxation in rat thoracic aorta in response to colforsin

BACKGROUND

Colforsin, a novel water-soluble forskolin derivative, increases intracellular cyclic AMP by direct stimulation of adenylate cyclase and has strong positive inotropic and vasodilative effects. However, it is not known whether colforsin causes nitric oxide (NO) release and enhances endothelium-dependent vascular relaxation.

METHODS

We studied NO production and relaxation on exposure to colforsin in thoracic aorta from rats aged 4, 12 and 60 weeks.

RESULTS

When a low concentration of colforsin was added to a solution bathing ring segments of aorta from 12-week-old rats, relaxation was greater in the ring segments with intact endothelium than in those from which the endothelium had been removed. A high concentration of colforsin induced the same degree of relaxation of ring segments with or without endothelium, probably by a direct effect on vascular smooth muscle cells. Production of NO in response to colforsin by cultured endothelial cells from 12-week-old rat aorta was demonstrated by the electron paramagnetic resonance spin trapping method. A low concentration of colforsin relaxed aortic segments with intact endothelium from 4-week-old rats more than those from 12-week-old or 60-week-old rats. Reversal of relaxation by NG-nitro L-arginine, an NO synthesis inhibitor, was most significant in arteries from 4-week-old rats. Production of NO after exposure to colforsin was greater in aortic segments from 4-week-old rats than older rats, as detected by an NO-selective electrode.

CONCLUSIONS

Colforsin induces vasodilation in part by releasing NO from the endothelium in rat thoracic aorta. In addition to a direct vasodilative effect on the vascular smooth muscle cells, an endothelium-dependent vasodilative effect is also important in younger arteries.

Effects of NKH477 on renal nerve stimulation-induced responses in anesthetized dogs

We evaluated the effects of an adenylate cyclase activator, N, N-dimetyl-beta-alanine[3R-(3alpha,4alphabeta,5beta+ ++,6beta,6aalpha, 10alpha,10abeta,10balpha)]-5(acetyloxy)-3-eth enyldodecahydro-10, 10b-dihydroxy-3,4a,7,7,10a-pentamethyl-1-oxo-1H-naphtho[2, 1-b]pyran-6-yl ester hydrochloride (NKH477), on neural control of renal functions in anesthetized dogs. Renal nerve stimulation (2 Hz) increased renal norepinephrine efflux and reduced renal blood flow, glomerular filtration rate, urine flow rate, urinary Na(+) excretion and fractional Na(+) excretion. Intrarenal arterial infusion of NKH477 (300 ng/kg/min) suppressed the stimulation-induced reductions in renal blood flow and glomerular filtration rate and attenuated the reductions in urine flow rate and urinary Na(+) excretion but not the changes in renal norepinephrine efflux and fractional Na(+) excretion. Infusion of NKH477 did not affect the urinary responses induced by renal nerve stimulation at a lower frequency (0.5-1 Hz) which had little influence on renal blood flow and glomerular filtration rate. The present results demonstrate that NKH477 inhibits renal vasoconstriction and hypofiltration but not the enhanced tubular Na(+) reabsorption during activation of the renal sympathetic nervous system.

Force, membrane potential and cytoplasmic Ca2+ responses to cyclic nucleotides in rat anococcygeus muscle

Simultaneous recordings of membrane potential and force, and cytoplasmic calcium ([Ca2+]i) and force were made in rat anococcygeus to determine whether membrane hyperpolarisation plays a role in cyclic nucleotide-induced relaxation. In the presence of phenylephrine (0.2 microM), which evoked sustained contraction, an elevation in [Ca2+]i, and depolarisation, nitroprusside (5 microM) caused 96+/-3% relaxation, 77+/-3% decrease in suprabasal [Ca2+]i, and 16+/-2 mV hyperpolarisation. Forskolin (1 microM) caused 98+/-1% relaxation, 92+/-2% decrease in suprabasal [Ca2+]i, and 18+/-1 mV hyperpolarisation. These responses persisted in the presence of a variety of K+ channel blockers or in ouabain. The decrease in [Ca2+]i preceded the commencement of relaxation whereas the onset of hyperpolarisation lagged behind. Thus, cyclic nucleotide-mediated relaxation in rat anococcygeus is not dependent on hyperpolarisation mediated by the opening of K+ channels. Rather, it is suggested that the decrease in [Ca2+]i gives rise to hyperpolarisation, which reflects a decline in the Ca2+ dependent conductance(s) activated by phenylephrine.

Vasorelaxant effect of olprinone, an inhibitor of phosphodiesterase 3, on mesenteric small artery and vein of rabbits

The effects of olprinone, a cardiotonic agent that inhibits cyclic GMP (cGMP)-inhibited phosphodiesterase, was studied on isolated rabbit mesenteric small artery and vein. In the presence of indomethacin and propranolol, olprinone at concentrations of 10 nM to 10 microM and 1 microM to 100 microM relaxed norepinephrine-stimulated mesenteric artery and vein in a concentration-dependent manner, respectively. The relaxation was not endothelium-dependent in the artery. Removal of the endothelium, however, increased marginally the response of the vein to olprinone. Olprinone-induced relaxation was less pronounced in arteries contracted with high KCl solution + norepinephrine than in those contracted with norepinephrine alone. Nicardipine inhibited this attenuating effect of high KCl solution on the olprinone-induced relaxation. Olprinone (1 microM) enhanced the relaxation of artery and vein in response to a cAMP-increasing agent, 6-(3-dimethylaminopropionyl) forskolin (NKH477), but not to a cGMP- increasing agent, glyceryl trinitrate. Norepinephrine (10 microM) and caffeine (5 mM) elicited a transient, phasic contraction of the artery in Ca2+-free solution. Both olprinone and NKH477 attenuated more potently the norepinephrine-induced contraction than the caffeine-induced contraction. When norepinephrine (10 microM) and caffeine (5 mM) were successively applied in Ca2+-free solution, the contractile effect of caffeine was diminished compared to that in artery which had not been pretreated with norepinephrine. When the contraction in response to norepinephrine was partially attenuated by 1 microM olprinone, the following contraction evoked by caffeine was enlarged. It is concluded that olprinone relaxes the small artery more strongly than the vein via its direct action on smooth muscles. It is suggested that olprinone attenuates norepinephrine-induced contraction through inhibition of receptor-operated transmembrane Ca2+ influx and Ca2+ release from intracellular storage sites.

Effect of cilostazol, a phosphodiesterase type III inhibitor, on histamine-induced increase in [Ca2+]i and force in middle cerebral artery of the rabbit

1. The effect of cilostazol, an inhibitor of phosphodiesterase type III (PDE III), on the contraction induced by histamine was studied by making simultaneous measurements of isometric force and the intracellular concentration of Ca2+ ([Ca2+]i) in endothelium-denuded muscle strips from the peripheral part of the middle cerebral artery of the rabbit. 2. High K+ (80 mM) produced a phasic, followed by a tonic increase in both [Ca2+]i and force. Cilostazol (10 microM) did not modify the resting [Ca2+]i, but it did significantly decrease the tonic contraction induced by high K+ without a corresponding change in the [Ca2+]i response. 3. Histamine (3 microM) produced a phasic, followed by a tonic increase in both [Ca2+]i and force. Cilostazol (3 and 10 microM) significantly reduced both the phasic and tonic increases in [Ca2+]i and force induced by histamine, in a concentration-dependent manner. 4. Rp-adenosine-3':5'-cyclic monophosphorothioate (Rp-cAMPS, 0.1 mM), a PDE-resistant inhibitor of protein kinase A (and as such a cyclic AMP antagonist), did not modify the increases in [Ca2+]i and force induced by histamine alone, but it did significantly decrease the cilostazol-induced inhibition of the histamine-induced responses. 5. In Ca2+-free solution containing 2 mM EGTA, both histamine (3 microM) and caffeine (10 mM) transiently increased [Ca2+]i and force. Cilostazol (1-10 microM) (i) significantly reduced the increases in [Ca2+]i and force induced by histamine, and (ii) significantly reduced the increase in force but not the increase in [Ca2+]i induced by caffeine. 6. In ryanodine-treated strips, which had functionally lost the histamine-sensitive Ca2+ storage sites, histamine (3 microM) slowly increased [Ca2+]i and force. Cilostazol (3 and 10 microM) lowered the resting [Ca2+]i, but did not modify the histamine-induced increase in [Ca2+]i, suggesting that functional Ca2+ storage sites are required for the cilostazol-induced inhibition of histamine-induced Ca2+ mobilization. 7. The [Ca2+]i-force relationship was obtained in ryanodine-treated strips by applying ascending concentrations of Ca2+ (0.16-2.6 mM) in Ca2+-free solution containing 100 mM K+. Histamine (3 microM) shifted the [Ca2+]i-force relationship to the left and increased the maximum Ca2+-induced force. Under the same conditions, whether in the presence or absence of 3 microM histamine, cilostazol (3-10 microM) shifted the [Ca2+]i-force relationship to the right without producing a change in the maximum Ca2+-induced force. 8. It is concluded that, in smooth muscle of the peripheral part of the rabbit middle cerebral artery, cilostazol attenuates the histamine-induced contraction both by inhibiting histamine-induced Ca2+ mobilization and by reducing the myofilament Ca2+ sensitivity. It is suggested that the increase in the cellular concentration of cyclic AMP that will follow the inhibition of PDE III may play an important role in the cilostazol-induced inhibition of the histamine-contraction.

Relaxant effects of NKH477, a new water-soluble forskolin derivative, on guinea-pig tracheal smooth muscle: the role of Ca2+-activated K+ channels

1. Mechanisms underlying the bronchorelaxant action of NKH477, a newly developed water-soluble forskolin derivative, were investigated in guinea-pig isolated tracheal smooth muscle. 2. In muscles precontracted with 3 microM histamine, NKH477 (1 nM-1 microM) caused a concentration-dependent decrease of isometric tension, resulting in a complete relaxation at 300 nM. The EC550 for the relaxation was 32.6+/-4.3 nM (n=6). 3. In the presence of 30 or 90 nM iberiotoxin (IbTX), a selective blocker of the large-conductance Ca2+-activated K+ (BK(Ca)) channel, the relaxing action of NKH477 on the histamine-induced contraction was inhibited, giving rise to a parallel shift of the concentration-response curves; the EC50 of NKH477 was increased to 131.4+/-20.4 nM at 30 nM IbTX (n=4), and 125.3+/-12.2 nM at 90 nM IbTX (n=4). 4. Pretreatment of muscles with 30 mM tetraethylammonium (TEA) caused a similar rightward shift of the concentration-response curve to NKH477 with an increase of the EC50 to 139.8+/-18.4 nM (n=5). In contrast, the relaxing action of NKH477 was unaffected by 10 microM glibenclamide, an ATP-sensitive K channel blocker, or by 100 nM apamin, a blocker of small conductance Ca2+-activated K+ channels. 5. In muscles pretreated with 1 microM nifedipine, a blocker of the voltage-dependent Ca2+ channel (VDC), 30-90 nM IbTX did not affect the relaxant effects of NKH477 on the histamine-induced contraction. 6. In muscles precontracted by a K+-rich (40 mM) solution, NKH477 caused only minimal relaxation (19.8+/-1.7%, n=4) even at the highest concentration (1 microM). 7. In experiments to measure the ratio of fura-2 fluorescence signals (R(340/380)) as an index of the intracellular Ca2+ concentration ([Ca2+]i), the application of 100 nM NKH477 or 200 nM isoprenaline to the preparation precontracted by 3 microM histamine resulted in a decrease in [Ca2+]i in association with a decrease in tension. The reduction of [Ca2+]i and tension by NKH477 was 47.0+/-5.6% and 62.8+/-7.0%, respectively (n=5), and that with isoprenaline 60.6+/-7.4% and 67.4+/-6.4%, respectively (n=5). These effects of NKH477 and isoprenaline on [Ca2+]i and tension were inhibited by 30 nM IbTX. The inhibitory action of IbTX was abolished in the presence of 1 microM nifedipine. 8. These results suggest that the bronchorelaxant action of NKH477 may result, at least in part, from activation of BK(Ca) channels, which may cause a hyperpolarization of smooth muscle cell membranes and a secondary decrease in Ca2+ influx through VDCs, leading to a decrease in [Ca2+]i.

Force and intracellular Ca2+ during cyclic nucleotide-mediated relaxation of rat anococcygeus muscle and the effects of cyclopiazonic acid

1. Simultaneous recordings of tension and [Ca2+]i were made in rat anococcygeus muscle strips to investigate possible mechanisms involved during cyclic nucleotide-mediated relaxation. Relaxation of pre-contracted muscles was induced by sodium nitroprusside (SNP) or forskolin and the effects of cyclopiazonic acid (CPA) on these responses were examined. 2. In muscles pre-contracted with 0.2 microM phenylephrine addition of SNP (10 microM) caused a rapid and near complete relaxation of force. This was accompanied by a decrease in [Ca2+]i, however, this was not of a comparable magnitude to the decrease in force. The level of [Ca2+]i in muscles relaxed with SNP was shown to be associated with substantially higher force levels in the absence of SNP. Forskolin (10 microM) caused a slower, essentially complete relaxation which was associated with a proportional decrease in [Ca2+]i. 3. In muscles pretreated with SNP or forskolin subsequent responses to phenylephrine were attenuated with both force and [Ca2+]i rising slowly to attain eventually levels similar to those observed when the relaxant was applied to pre-contracted muscles. 4. Exposure of the muscles to the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor, CPA (10 microM), resulted in a sustained increase in [Ca2+]i which, in most cases, was not associated with any force development. The relaxation and decrease in [Ca2+]i in response to both SNP and forskolin were attenuated and substantially slowed in the presence of CPA. Overall the extent of this attenuation was greater for SNP. For both SNP and forskolin, CPA attenuated the decrease in [Ca2+]i to a greater extent than the decrease in force. In some cases, SNP-mediated relaxation in the presence of CPA was observed with almost no detectable change in [Ca2+]i. 5. The results suggest that, in the rat anococcygeus muscle under normal circumstances, a lowering of [Ca2+]i can fully account for the relaxation induced by forskolin but not for that induced by SNP, where mechanisms independent of changes in [Ca2+]i appear to contribute. Whilst Ca2+ sequestration into the sarcoplasmic reticulum plays a role in the relaxation mediated by both SNP and forskolin other Ca2+ lowering mechanisms may also be involved, especially in the response to forskolin.

The effect of forskolin on 5-HT1-like and angiotensin II-induced vasoconstriction and cyclic AMP content of the rabbit isolated femoral artery

1. A characteristic feature of vasoconstrictor 5-HT1-like receptors in vitro is that responses mediated by these receptors are enhanced by other vasoconstrictor agents. In the present study, we have examined the influence of cellular cyclic AMP on vasoconstrictor responses to activation of 5-HT1-like receptors in isolated ring segments of the rabbit femoral artery (RbFA), and determined whether modulation of this second messenger underlies the ability of angiotensin II, an endogenous vasoconstrictor, to enhance 5-HT1-like responses. 2. In the presence of 0.1 microM ketanserin (to antagonize 5-HT2-receptors) and 0.3 microM prazosin (to antagonize alpha 1-adrenoceptors), 5-HT produced a concentration-related contraction, which was significantly augmented by pre-contraction of the vessel with 0.1-0.45 nM ([A30]) angiotensin II. Responses to 5-HT in the presence of angiotensin II were inhibited by the 5-HT1-like/5-HT2 antagonist, metergoline (1 microM). 3. The directly-acting adenylyl cyclase activator, forskolin (1 microM), abolished responses to angiotensin II and caused a rightward shift and concomitant depression of the 5-HT concentration-effect (E/[A]) curve. Higher concentrations of forskolin (> 10 microM) abolished responses to 5-HT and 1 microM sodium nitroprusside abolished responses to 5-HT and angiotensin II (n = 7). 4. In the presence of angiotensin II (0.1-0.45 nM), however, 1 microM forskolin failed to inhibit 5-HT-induced contractions; the E/A curve for 5-HT (in the presence of forskolin and angiotensin II) was not significantly different from that produced in the presence of angiotensin II alone. Similarly, the presence of angiotensin II (0.1-0.45 nM) was also able to overcome partially the inhibitory effect of 1 microM sodium nitroprusside against 5-HT-induced contractions (n = 7). In marked contrast, 5-HT failed to elicit a contraction in the presence of angiotensin II and 10 microM forskolin (n = 5). 5. 5-HT (1 microM) significantly reduced basal cyclic AMP accumulation by 35%, whereas angiotensin II (0.45 nM) was without effect. The combination of angiotensin II and 5-HT failed to alter significantly the reduction in cyclic AMP produced by 5-HT alone. Forskolin (1 microM) increased cyclic AMP levels 7 fold above basal, but neither 1 microM 5-HT nor a combination of 1 microM 5-HT and 0.45 nM angiotensin II produced a significant decrease in cyclic AMP content. 6. Whilst moderate concentrations of forskolin can inhibit the responses to either agent, simultaneous activation of angiotensin II and 5-HT1-like receptors can overcome the inhibitory effect of elevated levels of cyclic AMP. Since the potentiating effect of angiotensin II, in either the presence or absence of forskolin, occurs without significant alteration of cellular cyclic AMP, it seems likely that a cyclic AMP-independent pathway is implicated in the synergistic interaction between angiotensin II and vasoconstrictor 5-HT1-like receptors.

Pharmacological characterization of noradrenaline-induced contractions of the porcine isolated palmar lateral vein and palmar common digital artery

1. The aim of this study was to examine the pharmacological characteristics of alpha-adrenoceptor-mediated contractions in two porcine isolated blood vessels, the palmar lateral vein (PLV) and the palmar common digital artery (PCDA). This was carried out with noradrenaline used as the agonist throughout, and either phentolamine (non-selective alpha-adrenoceptor antagonist), prazosin and YM-12617 (selective alpha 1-adrenoceptor antagonists) or rauwolscine and CH-38083 (selective alpha 2-adrenoceptor antagonists). 2. Noradrenaline (0.003-10 microM) produced concentration-dependent contractions in both vessels, with the PCDA (pD2 = 6.33 +/- 0.07, n = 10) being approximately 10 fold less sensitive to noradrenaline compared to the PLV (pD2 = 7.39 +/- 0.09, n = 8). Also, the maximal response to noradrenaline was greater in the PCDA compared to the PLV. Phentolamine (0.03-30 microM) produced parallel rightward shifts in the CRC to noradrenaline in both tissue preparations. The pA2 values were similar and slopes of the Schild plots were not significantly different from unity, indicating an interaction between phentolamine and a single receptor in each preparation. 3. In the PCDA the alpha 1-adrenoceptor antagonists, prazosin (0.01-1 microM) and YM-12617 (0.01-1 microM) produced non-parallel rightwards shifts in the CRC to noradrenaline, with the lower 10-15% of the CRC exhibiting greater resistance to the effects of these antagonists compared to the upper part. In contrast, rauwolscine (1-10 microM) and CH-38083 (10 microM) produced parallel displacement of the CRC to noradrenaline. In the PLV, low concentrations of either alpha l- (0.01 microM) or alpha2-adrenoceptor antagonists(0.1-1 microM) produced a large shift in the CRC, but subsequent higher concentrations had only small additional effects. Based upon pKB values estimated from the effects of the lower concentrations of antagonists, the results are consistent with a large population of alpha1-adrenoceptors in the PCDA and a mixture of alpha l- and alpha2-adrenoceptors in the PLV.4. In both tissues, when an ac,- and an a2-adrenoceptor antagonist were used in combination the effect produced was greater than that with either agent alone. In contrast, the combination of the alpha1-adrenoceptor antagonists (prazosin and YM-12617 together) or the alpha2-adrenoceptor antagonists (CH-38083 and rauwolscine together) were no more effective than that produced by the individual antagonists. These findings suggest the presence of functional alpha l- and alpha2-adrenoceptors in the PLV andPCDA.5. Phenoxybenzamine (0.3-3 microM, 60min exposure) produced a concentration-dependent reduction in the maximal response to noradrenaline which was more pronounced in the PCDA than the PLV. After a 60 min exposure to a combination of phenoxybenzamine (1 microM) and rauwolscine (1 microM), the remaining NA-induced contraction after washout was resistant to prazosin (0.1 microM) and sensitive to rauwolscine(1 microM) in both tissue preparations, indicating the existence of functional alpha2-adrenoceptors in both vessels.6. Evidence suggests that post-junctional alpha l- and alpha2-adrenoceptors contribute to noradrenaline-induced contractions in the PCDA and PLV, with the latter possessing a larger population of functional alpha2-adrenoceptors.

Examination of the role of inhibition of cyclic AMP in alpha 2-adrenoceptor mediated contractions of the porcine isolated palmar lateral vein

1. We have examined the effect of elevation of cellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on alpha 1- and alpha 2-adrenoceptor-mediated contraction of the isolated palmar lateral vein of the pig. Cellular cyclic AMP was increased by either inhibition of phosphodiesterase by rolipram, or direct activation of adenylyl cyclase by forskolin. 2. Noradrenaline (1 nM-10 microM) caused concentration-dependent contractions of the porcine isolated palmar lateral vein (pD2 7.32 +/- 0.07, n = 10). The selective alpha 1-adrenoceptor antagonist, prazosin (0.1 microM) and the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM) caused a 10 fold rightward displacement of the concentration-response curve and a combination of the two antagonists caused a 200 fold rightward displacement of the concentration-response curve. The selective alpha 2-adrenoceptor agonist, UK-14304, also produced concentration-dependent contractions of the palmar lateral vein (pD2 7.70 +/- 0.15, n = 5), but the maximum response was 55.5 +/- 7.6% (n = 5) of that produced by noradrenaline. Prazosin (0.1 microM) failed to affect responses to UK-14304 but rauwolscine, 1 microM, caused a 200 fold rightward displacement. The estimated pKB value for rauwolscine (8.28 +/- 0.19, n = 10) is consistent with inhibition of alpha 2-adrenoceptors. Thus, the porcine isolated palmar lateral vein has a population of alpha 1- and alpha 2-adrenoceptors capable of producing a contraction. 3. Rolipram, 10 micro M, and forskolin, 1 micro M, caused a 2-3 fold rightward displacement of the noradrenaline concentration-response curve (CRC), but 1,9-dideoxyforskolin, 1 micro M, a forskolin analogue which does not activate adenylyl cyclase, failed to produce a significant inhibition of noradrenaline induced contractions. The combination of forskolin (1 micro M) and rolipram (10 micro M) were additive, producing a 20 fold rightward displacement of the noradrenaline CRC.4. Responses to noradrenaline were similarly affected by a combination of rolipram (10 micro M) and prazosin (0.1 micro M) (isolation of alpha 2-adrenoceptors) and the combination of rolipram (10 micro M) and rauwolscine(1 micro M) (isolation of alpha l-adrenoceptors), resulting in a 100 fold rightward displacement of the noradrenaline CRC. Although forskolin inhibited both alpha l- and alpha 2-adrenoceptor-mediated contractions,the effects produced were not similar. In particular, noradrenaline, 0.3-3 micro M, produced a significant contraction in the presence of forskolin (1 micro M) and prazosin (0.1 micro M) (an alpha 2-adrenoceptor-mediated response) but not in the presence of forskolin (1 micro M) and rauwolscine (1 micro M) (an alpha l-adrenoceptor mediated response).5. Five minute exposure to either rolipram (10 micro M) or forskolin (1 micro M) elevated [3H]-cyclic AMP of the porcine isolated palmar lateral vein by approximately 70% and 150-200%, respectively. Neither noradrenaline (1 nM- 100 micro M) nor UK-14304 (1 nM- 100 micro M) affected basal levels of [3H]-cyclic AMP,but both produced a concentration-dependent inhibition of forskolin-stimulated [3H]-cyclic AMP accumulation with a pKi of 7.43 +/- 0.1 (n = 3) and 7.97 +/- 0.18 (n = 3), respectively. The effect of noradrenaline against forskolin-stimulated [3H]-cyclic AMP accumulation was reversed by rauwolscine(1 micro M) but not by prazosin (0.1 micro M). In contrast, alpha 2-adrenoceptor activation did not affect rolipram induced elevation of [3H]-cyclic AMP.6. These findings indicate that M2-adrenoceptor contractions of the porcine isolated palmar lateral vein are not produced by reduction in cellular cyclic AMP per se. It is proposed that this response involves a novel signal transduction mechanism. However, when cellular cyclic AMP has been elevated by agents that stimulate adenylyl cyclase, rather than through inhibition of phosphodiesterase, the ability of alpha 2-adrenoceptors to inhibit cyclic AMP formation may be of functional importance in vascular smooth muscle.

Ca2+ mobilization by caffeine in single smooth muscle cells of the rat tail artery

The fluorescent dye fura-2 was used to study the effects of caffeine on cytosolic free Ca2+ level ([Ca2+]i) in freshly isolated single cells from the rat tail artery. Caffeine caused a concentration-dependent transient increase in [Ca2+]i and shortening of the cell. At higher concentrations (> 2 mM), a tonic increase in [Ca2+]i was also observed. The caffeine-induced changes in [Ca2+]i were reproducible with repeated challenges, even though the cells had contracted due to previous exposure to caffeine. Removal of extracellular Ca2+ reduced the resting [Ca2+]i to about half and abolished the tonic Ca2+ increase to caffeine. The transient component was not significantly affected to the first caffeine challenge after Ca2+ removal, but was abolished to the second challenge. Ryanodine (10 microM) significantly inhibited the responses to caffeine while nifedipine and TMB-8-(8-(diethylamino)octyl ester of 3,4,5-trimethoxybenzoic acid) were not effective. Thapsigargin (10-100 microM) induced a sustained increase in [Ca2+]i to 67 nM. The response of caffeine was not affected by thapsigargin. Pretreatment of the cells with noradrenaline (10 microM) abolished subsequent response to caffeine. These results show that Ca2+ responses to caffeine in single cells from the rat tail artery are reproducible with repeated caffeine challenge. Therefore, single cells can be used for comparison studies of the effects of pharmacological agents.