Effects of NKH477 on endothelin-1-induced renal responses in anaesthetized dogs

1. Intrarenal arterial infusion of a direct adenylate cyclase activator (NKH477; 300 ng/kg per min) increased renal blood flow, urine flow rate and urinary sodium excretion in anaesthetized dogs. 2. Intrarenal arterial infusion of endothelin (ET)-1 (2 ng/kg per min) reduced basal values of these parameters and glomerular filtration rate, which were recovered by the addition of NKH477 during ET-1 infusion. 3. These results demonstrate that NKH477 can counteract ET-1-induced antinatriuresis, mainly by restoring glomerular filtration.

Volume sensitive efflux of taurine in HEK293 cells overexpressing phospholemman

The role of the phospholemman (PLM) on the efflux of taurine and chloride induced by swelling was studied in HEK293 cells overexpressing stable transfected PLM. PLM, a substrate for protein kinases C and A, is a protein that induces an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. In PLM-overexpressing cells the process of RVD was more rapid and efficient (75%) than in control cells (44%). Also, [(3)H]taurine and (125)I efflux induced by hyposmolarity were markedly increased (30-100%) in two subclones of cells overexpressing PLM. This increased efflux was sensitive to the Cl channel blockers DDF, NPPB and DIDS. Acute treatment of control cells with isoproterenol and norepinephrine induced a significant potentiation (50-60%) of [(3)H]taurine release induced by hyposmolarity. In PLM-overexpressing cells the potentiation by these drugs was higher (100%). Insulin induced also an increase in [(3)H]taurine release, but only in PLM-overexpressing cells (50%). These results indicate that PLM may play a role in the RVD and that its phosphorylation may have a physiological significance during this process. The mechanisms involved in this process could include the activation of PLM itself as channel or the modulation of other preexisting channels.

Adenylate cyclase and potassium channels are involved in forskolin- and 1,9-dideoxyforskolin-induced inhibition of pregnant rat uterus contractility

OBJECTIVE

We sought to study the contribution of potassium channels in the effect of forskolin and 1,9-dideoxyforskolin on uterine contractility in the pregnant rat.

STUDY DESIGN

Rings taken from the middle portions of uterine horns from rats at 16 days of gestation were positioned in organ chambers containing physiologic salt solution bubbled with 5% carbon dioxide in air (37 degrees C, pH approximately 7.4) for isometric tension recording under 2 g passive tension. The effects of cumulative concentrations of forskolin and 1,9-dideoxyforskolin in the absence or presence of an adenylate cyclase inhibitor (MDL-12,330A, 10(-5) mol/L), a nonselective potassium channel blocker (tetrabutylammonium, 10(-4) mol/L), or an adenosine triphosphate-dependent potassium channel blocker (glibenclamide 10(-5) mol/L) were studied.

RESULTS

Both forskolin and, to a lesser extent, 1,9-dideoxyforskolin inhibit uterine contractions. Tetrabutylammonium, glibenclamide, and MDL-12, 330A attenuated the effects of forskolin, whereas glibenclamide was less effective against 1,9-dideoxyforskolin.

CONCLUSION

Activation of adenylate cyclases, as well as adenosine triphosphate-dependent potassium channels and, to a greater extent, calcium-dependent potassium channels, is involved in the inhibitory effect of forskolin in uterine rings from rats at 16 days of gestation. Inhibition of uterine contractions by 1,9-dideoxyforskolin is less than that by forskolin and involves activation of adenylate cyclase and calcium-dependent potassium channels. Whether activation of guanylate cyclase is involved in the effect of the agents on calcium-dependent potassium channels needs further investigation. 1, 9-Dideoxyforskolin is not an inactive isomer of forskolin in rat uterine rings.

Effect of forskolin on acetylcholine-induced current in rat pheochromocytoma cells

AIM

To study the effect of forskolin on the nicotinic receptor (NicR) of PC12 cells.

METHODS

The acetylcholine (ACh)-induced current (IACh) was measured on PC12 cells by whole-cell clamp technique.

RESULTS

The IACh could be blocked by d-tubocurarine chloride and atropine had no effect on IACh. Infusion of forskolin (1-50 mumol.L-1) caused an inhibition on IACh, which was reversible, concentration-dependent, and voltage-independent. Preincubation with 8-bromo-adenosine-3', 5'-adenosine monophosphate (8-Br-cAMP), a cell-permeable cAMP analog which preferentially activated cyclic AMP-dependent protein kinase (CADPK), for 20 min, did not affect the IACh and the inhibitory effect of forskolin. Infusion of 1,9-dideoxyforskolin, an analog of forskolin which did not activate adenyl cyclase, also caused an inhibition on IACh.

CONCLUSION

The inhibitory effect of forskolin on IACh in PC12 cells is not mediated by activating the adenyl cyclase. Probably, the lipophilic forskolin acts via perturbing the plasma membrane lipid structure and altering the function of the NicR.

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.

Prior short-term synaptic disinhibition facilitates long-term potentiation and suppresses long-term depression at CA1 hippocampal synapses

Long-term potentiation (LTP) and long-term depression (LTD) are two main forms of activity-dependent synaptic plasticity that have been extensively studied as the putative mechanisms underlying learning and memory. Current studies have demonstrated that prior synaptic activity can influence the subsequent induction of LTP and LTD at Schaffer collateral-CA1 synapses. Here, we show that prior short-term synaptic disinhibition induced by type A gamma-aminobutyric acid (GABA) receptor antagonist picrotoxin exhibited a facilitation of LTP induction and an inhibition of LTD induction. This effect lasted between 10 and 30 min after washout of picrotoxin and was specifically inhibited by the L-type voltage-operated Ca2+ channel (VOCC) blocker nimodipine, but not by the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphopentanoic acid (D-APV). Moreover, this picrotoxin-induced priming effect was mimicked by forskolin, an activator of cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA), and was blocked by the adenylyl cyclase inhibitor 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22536) and the PKA inhibitor Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS). It was also found that following picrotoxin application, CA1 neurons have a higher probability of synchronous discharge in response to a population of excitatory postsynaptic potential (EPSP) of fixed slope (EPSP/spike potentiation). However, picrotoxin treatment did not significantly affect paired-pulse facilitation (PPF). These findings suggest that a brief of GABAergic disinhibition can act as a priming stimulus for the subsequent induction of LTP and LTD at Schaffer collateral-CA1 synapses. The increase in Ca2+ influx through L-type VOCCs in turn triggering a cAMP/PKA signalling pathway is a possible molecular mechanism underlying this priming effect.

Sequential changes in autonomic regulation of cardiac myocytes after in vivo endotoxin injection in rat

We report that in vivo injection of endotoxin (EDTX, 6 mg. kg(-)(1)) induces cardiovascular alterations in rats that closely mimic the clinical situation, as assessed by in vivo hemodynamic measurements in anesthetized and conscious, chronically instrumented animals. The patch-clamp technique was used to characterize the L-type calcium current (I(Ca)) and its autonomic regulation in isolated cardiac myocytes. The density of I(Ca) progressively decreased at 12 and 36 h after EDTX injection. However, the dihydropyridine (+/-)Bay K 8644 (100 nM) enhanced I(Ca) to levels similar to those in control and EDTX-treated myocytes. In addition, the net stimulatory effect of a beta-adrenergic agonist (isoproterenol) on I(Ca) was increased 12 h after EDTX injection. This change in the beta-adrenergic effect declined 24 h later. The potentiation in the beta-adrenergic stimulation of I(Ca) was mimicked by L858051 (10 microM), a direct activator of adenylyl cyclase, but not by IBMX (200 microM), a phosphodiesterase inhibitor. Besides, the antiadrenergic effect of acetylcholine on I(Ca) was unchanged 12 h after EDTX injection, but increased 36 h after EDTX injection. These results support the hypothesis that time-dependent changes in the adenylyl cyclase pathway in cardiac myocytes may contribute, via the autonomic regulation of I(Ca), to the severity of myocardial dysfunction during sepsis.

Anion channel blockers differentially affect T-type Ca(2+) currents of mouse spermatogenic cells, alpha1E currents expressed in Xenopus oocytes and the sperm acrosome reaction

The direct electrophysiological characterization of sperm Ca(2+) channels has been precluded by their small size and flat shape. An alternative to study these channels is to use spermatogenic cells, the progenitors of sperm, which are larger and easier to patch-clamp. In mouse and rat, the only voltage-dependent Ca(2+) currents displayed by these cells are of the T type. Because compounds that block these currents inhibit the zona pellucida-induced Ca(2+) uptake and the sperm acrosome reaction (AR) at similar concentrations, it is likely that they are fundamental for this process. Recent single channel recordings in mouse sperm demonstrated the presence of a Cl(-) channel. This channel and the zona pellucida (ZP)-induced AR were inhibited by niflumic acid (NA), an anion channel blocker [Espinosa et al. (1998): FEBS Lett 426:47-51]. Because NA and other anion channel blockers modulate cationic channels as well, it became important to determine whether they affect the T-type Ca(2+) currents of spermatogenic cells. These currents were blocked in a voltage-dependent manner by NA, 1, 9-dideoxyforskolin (DDF), and 5-nitro-2-(3-phenylpropylamine)benzoic acid (NPPB). The IC(50) values at -20 mV were 43 microM for NA, 28 microM for DDF, and 15 microM for NPPB. Moreover, DDF partially inhibited the ZP-induced AR (40% at 1 microM) and NPPB displayed an IC(50) value of 6 microM for this reaction. These results suggest that NA and DDF do not inhibit the ZP-induced AR by blocking T-type Ca(2+) currents, while NPPB may do so. Interestingly 200 microM NA was basically unable to inhibit alpha1E Ca(2+) channels expressed in Xenopus oocytes, questioning that this alpha subunit codes for the T-type Ca(2+) channels present in spermatogenic cells. Evidence for the presence of alpha1C, alpha1G, and alpha1H in mouse pachytene spematocytes and in round and condensing spermatids is presented.

[Cardiovascular effects of colforsin daropate hydrochloride, a novel drug for the treatment of acute heart failure]

Colforsin daropate hydrochloride (COL) is a water-soluble forskolin derivative for the treatment of acute heart failure. COL, like forskolin, stimulated adenylate cyclase (AC) directly and produced pharmacologic activities accompanied by the increase in cellular cAMP. COL was different from forskolin in water-solubility, duration of action, BBB permeability, oral activity and AC-subtype selectivity. COL was a inodilator with positive inotropic and vasodilator effects and was effective on a beta-receptor desensitized-heart model in which the effects of beta-agonists and PDE inhibitors were attenuated. COL improved cardiac function in some heart failure models. In the clinical studies, COL improved hemodynamics, subjective and objective symptoms of heart failure patients, and was also effective in the catecholamine-resistant heart failure patients. COL is a first clinically available adenylate cyclase activator. Further information from the post-marketing-surveillance will provide information that will enable more adequate usage of this drug.

Effect of protein kinase A-induced phosphorylation on the gating mechanism of the brain Na+ channel: model fitting to whole-cell current traces

The activity of the voltage-gated Na+ channel is subjected to modulation through covalent modifications. It has been previously shown that brain Na+ currents are reduced following the activation of the protein kinase A (PKA) pathway, but the effect of the phosphorylation on the gating mechanism of the channel has not been demonstrated so far. In this study, we analyze the whole-cell Na+ current recorded in the absence or presence of forskolin, which stimulates the PKA pathway. A minimal molecular model of the gating mechanism of the Na+ channel is defined to fit the experimental data: it consists of three closed states, one open state, and two inactivated states. We experimentally demonstrate that the kinetics of inactivation from the closed states are not affected by phosphorylation. The results obtained by computer fitting indicate that, among all the kinetic parameters describing the transitions between states, only one parameter is significantly modified in the presence of forskolin, and corresponds to the acceleration of the inactivation from the open state. This conclusion is supported by the analysis of current traces obtained from cells in the presence of a phosphatase inhibitor or loaded with the PKA catalytic unit, and is in agreement with previously reported single channel records.

Voltage-activated K+ channels and membrane depolarization regulate accumulation of the cyclin-dependent kinase inhibitors p27(Kip1) and p21(CIP1) in glial progenitor cells

Neural cell development is regulated by membrane ion channel activity. We have previously demonstrated that cell membrane depolarization with veratridine or blockage of K+ channels with tetraethylammonium (TEA) inhibit oligodendrocyte progenitor (OP) proliferation and differentiation (); however the molecular events involved are largely unknown. Here we show that forskolin (FSK) and its derivative dideoxyforskolin (DFSK) block K+ channels in OPs and inhibit cell proliferation. The antiproliferative effects of TEA, FSK, DFSK, and veratridine were attributable to OP cell cycle arrest in G1 phase. In fact, (1) cyclin D accumulation in synchronized OP cells was not affected by K+ channel blockers or veratridine; (2) these agents prevented OP cell proliferation only if present during G1 phase; and (3) G1 blockers, such as rapamycin and deferoxamine, mimicked the anti-proliferative effects of K+ channel blockers. DFSK also prevented OP differentiation, whereas FSK had no effect. Blockage of K+ channels and membrane depolarization also caused accumulation of the cyclin-dependent kinase inhibitors p27(Kip1) and p21(CIP1) in OP cells. The antiproliferative effects of K+ channel blockers and veratridine were still present in OP cells isolated from INK4a-/- mice, lacking the cyclin-dependent kinase inhibitors p16(INK4a) and p19(ARF). Our results demonstrate that blockage of K+ channels and cell depolarization induce G1 arrest in the OP cell cycle through a mechanism that may involve p27(Kip1) and p21(CIP1) and further support the conclusion that OP cell cycle arrest and differentiation are two uncoupled events.

Effects of NKH477 on renal functions and cyclic AMP production in anesthetized dogs

The present study was undertaken to evaluate the effects of an adenylate cyclase activator N,N-dimethyl-beta-alanine[3R-(3alpha, 4alphabeta, 5beta, 6beta, 6aalpha, 10alpha, 10abeta, 10balpha)]-5(acetyloxy)-3-ethenyldodecahydro-10, 10b-dihydroxy-3, 4a, 7, 7, 10a-pentamethyl-1-oxo-1H-naphtho [2,1-b] pyran-6-yl ester hydrochloride (NKH477) on renal functions and cyclic AMP production in the dog kidney. The intrarenal arterial infusion of NKH477 (30, 100 and 300 ng kg(-1) min(-1)) increased renal blood flow, glomerular filtration rate, urine flow rate, urinary Na+ and cyclic AMP excretion, fractional Na+ excretion and arterial and renal venous plasma cyclic AMP concentrations in a dose-dependent manner. The intrarenal arterial infusion of rolipram (0.3 microg kg(-1) min(-1)), a cyclic AMP-specific phosphodiesterase inhibitor, also caused the same renal responses as NKH477. The increasing effects of NKH477 on renal blood flow, fractional Na+ excretion and renal venous plasma cyclic AMP concentration were facilitated in the presence of rolipram. NKH477 reduced glomerular filtration rate and filtration fraction in the presence of rolipram. The increasing effects of NKH477 on urine flow rate and urinary Na+ excretion were not affected by rolipram. The present results suggest that NKH477 increases glomerular filtration and suppresses tubular sodium reabsorption through activation of cyclic AMP production, and thereby induces natriuresis. The results also demonstrate that renal cyclic AMP level during the activation of adenylate cyclase is regulated by phosphodiesterase IV in both the vascular and tubular sites.

Stimulation of adenylyl cyclase and induction of brain-derived neurotrophic factor and TrkB mRNA by NKH477, a novel and potent forskolin derivative

The present study was undertaken to examine whether NKH477, a novel and potent water-soluble forskolin derivative, stimulates adenylyl cyclase and regulates brain-derived neurotrophic factor (BDNF) and TrkB expression in the rat brain. Administration of NKH477 at a dose of 1.0 mg/kg, but not 0.1 mg/kg, increased levels of cyclic AMP (cAMP) in a time-dependent manner in frontal cortex and hippocampus. Repeated administration of NKH477 (1.0 mg/kg) for 7 or 14 days also increased levels of cAMP in these two brain regions, indicating that the response does not desensitize with chronic treatment. In addition, administration of NKH477 at the 1 mg/kg dose increased the expression of BDNF and TrkB mRNA in frontal cortex and hippocampus. This effect was observed after single, as well as repeated (7 or 14 days), administration of NKH477. These results demonstrate that NKH477 administration rapidly increases cAMP levels in brain and provides evidence that stimulation of this second messenger system increases the expression of BDNF and TrkB mRNA.

FlCRhR/cyclic AMP signaling in myenteric ganglia and calbindin-D28 intrinsic primary afferent neurons involves adenylyl cyclases I, III and IV

The aims of this study were to improve insight into cAMP signaling in myenteric neurons and glia and identify the adenylyl cyclase (AC) isoforms expressed in myenteric ganglia of the guinea-pig small intestine. An increase in the intracellular cAMP levels was measured indirectly by an increase in the 520 nm/580 nm fluorescence emission ratio of the protein kinase A fluorosensor FlCRhR. Forskolin or pituitary adenylyl cyclase activating peptide caused an increase in cAMP levels in cell somas and neurites and elicited a slow EPSP-like response in myenteric AH/Type 2 neurons, whereas the inactive form of forskolin was without these effects. Glia displayed similar cAMP responses. Immunoblot analysis showed that AC I, III and IV were present in myenteric ganglia, with AC I being detected as two bands of 160 kDa and 185 kDa, AC III as two bands near 220 kDa, and AC IV as two bands of greater than 220 kDa. Pretreatment with N-ethylmaleimide and N-glycosidase F revealed an AC IV band at 115 kDa. Preabsorption with specific blocking peptides prevented detection of AC I or AC IV immunoreactive proteins. In ganglia which expressed strong AC IV immunoreactivity, no immunoreactive bands were detected for AC II, AC V/VI, AC VII or AC VIII. The amount of AC isoforms expressed in myenteric ganglia followed the order of AC IV&z.Gt;III>I. Immunofluorescent labeling studies revealed that AC I, AC III and AC IV were variably expressed in myenteric neurons and glia of the duodenum, jejunum and ileum. In the guinea-pig ileum, AC I, III and IV immunoreactivities were respectively present in 26%, 58% and 89% of calbindin-D28-colabeled myenteric neurons. These findings suggest that (1) AC I, AC III and AC IV variably contribute to cAMP signaling in myenteric ganglia, (2) AC I, AC III and AC IV may be differentially expressed in distinct subsets of calbindin-D28 neurons which may represent intrinsic primary afferent myenteric neurons. Our study also provides direct evidence for activation of cAMP-dependent protein kinase.

Regulation of cyclic AMP level by progesterone in ovariectomized rat neocortex

Exposure of neocortical slices to progesterone, without prior treatment with estrogen, augmented forskolin-induced cyclic AMP within 15 min. 30 nM progesterone produced approximately 1/2 the maximal effect but as little as 10 nM progesterone produced a detectable increase in cyclic AMP. When forskolin was replaced by dideoxyforskolin, an analog that does not directly stimulate adenylyl cyclase but shares many of its other actions, progesterone did not augment cyclic AMP. Progesterone also failed to affect increased cyclic AMP that followed exposure to norepinephrine or isoproterenol. The effect of progesterone upon cyclic AMP was also evident when tetrodotoxin was added to block voltage-dependent sodium channels, suggesting that intercellular communication that is dependent upon action potentials was not necessary. The effect of progesterone was at least partially blocked by antagonists of GABAA receptor action, suggesting the involvement of GABAA or GABAA-like receptors. The effect of progesterone was also not homogeneous over the neo cortex. While forskolin-stimulated cyclic AMP was augmented by progesterone in the parietal and occipital regions, it was suppressed in the frontal region. These results are envisioned as a progesterone action upon a small and perhaps compartmentalized component of the cellular cyclic AMP system, an effect that is made detectable in our whole-tissue assay by the well known ability of forskolin to potentiate many hormonal effects upon cyclic AMP.

Characterization of positive inotropic effect of colforsin daropate [correction of dapropate] hydrochloride, a water-soluble forskolin derivative, in isolated adult rat cardiomyocytes

The present study was undertaken to characterize the positive inotropic action of colforsin daropate [corrected] hydrochloride (NKH477), a novel water-soluble forskolin derivative, on isolated cardiomyocytes of adult rats. Simultaneous measurements of cellular contraction and intracellular calcium concentration ([Ca2+]i) were carried out. The effects of isoprenaline and ouabain on these parameters were also determined for comparison. The contraction and maximum [Ca2+]i of NKH477-, isoprenaline-, or ouabain-treated cells were increased concentration dependently. Peak shortening of NKH477-treated cells was positively correlated with the shortening velocity and inversely with the time to peak shortening. Maximum, but not minimum, [Ca2+]i in NKH477-treated cells was correlated with the rate of increase in [Ca2+]i and inversely with the time to maximum [Ca2+]i. Similar results were obtained with isoprenaline. In contrast, ouabain increased both maximum and minimum [Ca2+]i. Treatment with either NKH477 or isoprenaline increased cellular cAMP content, but treatment with ouabain did not. These results suggest that the positive inotropic action of NKH477 is associated with an increase in [Ca2+]i and acceleration of its kinetics.

High affinity regulation of cardiac Cl- and Ca2+ conductances by (13R)-spiroforskolin

The effects of a new forskolin derivative, (13R)-spiroforskolin, on the ventricular cAMP-activated chloride current (I(Cl(cAMP))) and the atrial L-type calcium current (I(Ca,L)) were measured by means of whole-cell recording from isolated guinea-pig cardiac myocytes at 30 degrees C and 20-22 degrees C, respectively. In contrast to forskolin, the derivative contains a tetrahydrofuran rather than a tetrahydropyran moiety. (13R)-spiroforskolin activated I(Cl(CAMP)) in 58% of the ventricular myocytes studied. The concentration required for the half maximal effect (EC50 value) amounted to 9.6x10(-11) M and was lower than the EC50 value for forskolin (2.4x10(-8) M). (13R)-spiroforskolin evoked a smaller maximal I(Cl(cAMP)) amplitude than forskolin. The rundown of the (13R)-spiroforskolin-activated I(Cl(cAMP)) was faster than that of the forskolin-induced current. Neither forskolin nor (13R)-spiroforskolin in maximally effective concentrations increased I(Cl(cAMP)) in cells containing high concentrations of cAMP. Furthermore, as an activator of atrial I(Ca,L) (13R)-spiroforskolin displayed a smaller activation and a lower EC50 value (5.8x10(-10) M) than forskolin (EC50 value: 3.7x10(-7) M). The effect of (13R)-spiroforskolin was observed in only 30% of the atrial cells studied. None of the drugs exerted a stimulatory effect in atrial cells containing a high [cAMP]. The washout of the drug effect was significantly faster in (13R)-spiroforskolin- than in forskolin-treated atrial myocytes. We conclude that (13R)-spiroforskolin as a forskolin derivative displays unique characteristics. It is a more potent but less efficacious activator of cardiac ionic conductances than the parent compound. The results suggest that (13R)-spiroforskolin, like forskolin, most probably exerts its effects via stimulation of the adenylyl cyclase.

In search of novel water soluble forskolin analogues for positive inotropic activity

Using the novel lead from hydroxy acetyl substituted forskolin analogues, such as 7 beta-hydroxyacetyl-7 beta-deacetyl forskolin or 6 beta-hydroxyacetyl forskolin, a number of water soluble omega-amino acyl derivatives were synthesized. Two such compounds 6 and 18 showed better in vitro activity but failed to show in vivo activity.

cAMP-dependent long-term potentiation of nitric oxide release from cerebellar parallel fibers in rats

Nitric Oxide (NO) is released from parallel fibers (PFs) after PF stimulation. NO-cGMP signaling is essential for long-term depression (LTD) in cerebellar PF-Purkinje cell synapses, which also exhibit presynaptic long-term potentiation (LTP) after tetanic PF stimulation. This LTP is dependent on cAMP but not NO-cGMP signaling. In this study, we analyzed long-term changes of NO release from PFs in rat cerebellar slices using electrochemical NO probes. Repetitive PF stimulation at 10 Hz for 2 sec elicited a transient increase in NO concentration (2.2 +/- 0.1 nM; mean +/- SEM; n = 116). This NO release exhibited long-term potentiation (LTPNO) by 36 +/- 3% (n = 15) after tetanic PF stimulation. Induction of LTPNO was not affected by Glu receptor antagonists. NO release from PFs was also potentiated by L-Arg (ARG) (100 microM), forskolin (50 microM), and 8-bromo-cAMP (Br-cAMP) (1 mM) but not by 1,9-dideoxyforskolin (50 microM), a biologically inactive analog of forskolin. The potentiation induced by forskolin was significantly suppressed by H89 (10 microM), a blocker of cAMP-dependent protein kinase. The potentiation induced by forskolin, but not that induced by Arg, interfered with LTPNO. H89 (10 microM) and KT5720 (1 microM), another blocker of cAMP-dependent protein kinase, but not KT5823 (300 nM), a blocker of cGMP-dependent protein kinase, significantly suppressed LTPNO. These data indicate that neural NO release is under activity-dependent control, just as synaptic transmitter release is. LTPNO might play a role in cross talk between presynaptic and postsynaptic plasticity by facilitating NO-cGMP-dependent postsynaptic LTD after induction of cAMP-dependent presynaptic LTP and LTPNO.

Hydroxyacyl derivatives of forskolin--their positive inotropic activity

Using appropriate protection and deprotection sequence novel hydroxyacyl chains of the type CO(CH2)nOH are synthesized and are utilized to develop new analogues of forskolin. Several compounds showed good positive inotropic activity. Compound 12 is almost 10 times more active than forskolin (EC50 = 0.002 microgram/ml).].

Physiological and biochemical adrenergic regulation of the stunned myocardium

A dual approach was employed to study beta-adrenergic receptor signal transduction in post ischemic (stunned) myocardium, examining physiological interventions in awake, chronically instrumented pigs and biochemical, cellular mechanisms in sarcolemmal preparations from the stunned hearts using the contralateral non-ischemic zone as a control. Ten min of coronary artery occlusion (CAO) and 30 min coronary artery reperfusion (CAR) resulted in depressed posterior wall-thickening (myocardial stunning). Isoproterenol increased transmural wall thickening more in stunned myocardium than in non-ischemic myocardium. In contrast, the responses of wall thickening to forskolin, actually decreased during stunning compared with control. NKH 477, a water soluble forskolin derivative, that does not activate cardiac nerves, increased wall thickening in non-ischemic tissue similarly to the effects on stunned myocardium. Increasing cardiac neural tone reflexly with inferior venal caval occlusion (IVCO) elicited similar results to forskolin, i.e., stunned myocardium responded with less of an increase in wall thickening as compared with non-ischemic myocardium. Beta-adrenergic receptor density, as determined with 125I-cyanopindolol binding, was significantly increased in stunned subendocardium and subepicardium compared with respective values in non-ischemic myocardium. There were no differences in the response of adenylyl cyclase to isoproterenol in stunned and non-ischemic myocardium. The enhanced responsiveness of the beta-adrenergic receptor to isoproterenol stimulation in stunned myocardium corresponded to the increase in beta-adrenergic receptor density. The combination of enhanced responses to isoproterenol, and decreased responses to forskolin and to IVCO and preserved responsiveness to NKH 477, suggest that stunned myocardium is characterized by transient sympathetic neural stunning. The enhanced sensitivity to beta-adrenergic receptor stimulation has important clinical implications, both in terms of therapy of stunned myocardium and detection of stunned and/or hibernating myocardium, i.e., low dose dobutamine echocardiography.

Osmosensitive C1- currents and their relevance to regulatory volume decrease in human intestinal T84 cells: outwardly vs. inwardly rectifying currents

1. The swelling-activated outwardly rectifying Cl- current (ICl(swell)) recorded in T84 human intestinal cells was completely blocked by 10 microM tamoxifen, while 300 microM Cd2+ had no effect. 2. A ClC-2-like, inwardly rectifying Cl- current was activated after strong hyperpolarization in T84 cells. This current was completely inhibited by 300 microM Cd2+, unaffected by 10 microM tamoxifen, and its magnitude increased slightly in response to cell swelling under hyposmotic conditions. However, the swelling-dependent modulation occurred only after prior activation by hyperpolarizing voltages. 3. T84 cells behaved initially close to perfect osmometers in response to changes in external osmolalities between +20 and -30 %. The cells underwent full regulatory volume decrease (RVD) within 16 min when exposed to 30 or 10 % hyposmotic shocks. 4. Pharmacological tools were used to determine the anionic pathway(s) involved in RVD in T84 cells. Tamoxifen (10 microM), 1,9-dideoxyforskolin (DDFSK; 100 microM) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 100 microM) blocked RVD while 300 microM Cd2+ had no effect upon RVD following a 30 % hyposmotic shock. The RVD response was similarly unaffected by Cd2+ when cells were exposed to a smaller (10 %) hyposmotic shock. 5. In conclusion, these data show that the anionic pathway primarily activated by cell swelling and relevant to RVD in T84 cells is the tamoxifen-, DDFSK- and DIDS-sensitive ICl(swell) and not the hyperpolarization-activated, Cd2+-sensitive Cl- current associated with the ClC-2 Cl- channel.

Eating induced by perifornical cAMP is behaviorally selective and involves protein kinase activity

It has previously been shown that agents that increase endogenous cAMP elicit robust eating when injected into the perifornical hypothalamus (PFH) but not when injected into surrounding brain sites, suggesting that PFH cAMP may play a role in eating control. We report here that bilateral microinjection of the adenylyl cyclase activator 7-deacetyl-7-O-(N-methylpiperazino)-gamma-butyryl-forskolin dihydrochloride (MPB forskolin; 300 nmol/0.3 microl) into the PFH is sufficient to elicit intense eating (up to 15.7 +/- 2.3 g in 2 h) in satiated rats, without concomitant effects on other behaviors, including gnawing and drinking. In contrast, the inactive analog 1, 9-dideoxyforskolin is ineffective, suggesting that the effects of MPB forskolin are behaviorally selective and pharmacologically specific. We also show that injection of the protein kinase A inhibitor H-89 (100 nmol) into the PFH reduced MPB forskolin-induced eating by up to 50%. Collectively, these results suggest that increased cAMP production in a single brain area may be sufficient to selectively generate a patterned, goal-oriented behavior by activating cAMP-dependent protein kinase.

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.