Effects of forskolin in rat vas deferens--evidence for facilitatory beta-adrenoceptors

Novel model for "calcium paradox" in sympathetic transmission of smooth muscles: role of cyclic AMP pathway

It is well established that reduction of Ca2+ influx through L-type voltage-dependent Ca2+ channel (L-type VDCC), or increase of cytosolic cAMP concentration ([cAMP]c), inhibit contractile activity of smooth muscles in response to transmitters released from sympathetic nerves. Surprisingly, in this work we observed that simultaneous administration of L-type VDCC blocker (verapamil) and [cAMP]c enhancers (rolipram, IBMX and forskolin) potentiated purinergic contractions evoked by electrical field stimulation of rat vas deferens, instead of inhibiting them. These results, including its role in sympathetic transmission, can be considered as a "calcium paradox". On the other hand, this potentiation was prevented by reduction of [cAMP]c by inhibition of adenylyl cyclase (SQ 22536) or depletion of Ca2+ storage of sarco-endoplasmic reticulum by blockade of Ca2+ reuptake (thapsigargin). In addition, cytosolic Ca2+ concentration ([Ca2+]c) evaluated by fluorescence microscopy in rat adrenal medullary slices was significantly reduced by verapamil or rolipram. In contrast, simultaneous incubation of adrenal slices with these compounds significantly increased [Ca2+]c. This effect was prevented by thapsigargin. Thus, a reduction of [Ca2+]c due to blockade of Ca2+ influx through L-type VDCC could stimulate adenylyl cyclase activity increasing [cAMP]c thereby stimulating Ca2+ release from endoplasmic reticulum, resulting in augmented transmitter release in sympathetic nerves and contraction.

Pathways involved in muscarinic M1 and M2 receptor stimulation in rabbit vas deferens

Pretreatment of the field-stimulated rabbit isolated vas deferens for 30 min with LiCl (2 x 10(-2) and 4 x 10(-2) M) attenuated the inhibition of neurogenic twitch contractions due to muscarinic M1 receptor stimulation by 4-(4-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343), and enhanced the muscarinic M2 receptor-mediated potentiation of contractions evoked by carbachol. When the tissues were preincubated for 5 min with the adenylate cyclase activator, forskolin (3 x 10(-8)-3 x 10(-7) M), the response to carbachol was attenuated whereas that to 4-Cl-McN-A-343 remained unchanged. 1,9-Dideoxy-forskolin (3 x 10(-7) and 10(-6) M), which fails to activate cyclase, did not abolish the carbachol effect. In addition, desensitization of the response to 4-Cl-McN-A-343 but not to carbachol occurred in preparations incubated for 90 min with the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA, 3 x 10(-8)-3 x 10(-7) M), whereas its inactive 4 alpha-stereoisomer (4 alpha-PMA, 3 x 10(-7) M) was without effect. In unstimulated preparations, LiCl, forskolin and PMA did not impair contractions due to exogenous ATP (10(-3) M). These findings are consistent with the hypothesis that, in rabbit vas deferens, inhibitory muscarinic M1 receptors stimulate LiCl-sensitive phosphatidylinositol turnover (IP3 pathway) involving protein kinase C, whilst excitatory muscarinic M2 receptors are coupled to inhibition of adenylate cyclase, resulting in reduced levels of cyclic AMP.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Prejunctional beta-adrenoceptors, angiotensin II and neuropeptide Y receptors on sympathetic nerves in mouse atria are linked to N-ethylmaleimide-susceptible G-proteins

We used the alkylating agent N-ethylmaleimide in order to investigate G-proteins linked to release-modulating prejunctional receptors of sympathetic nerves in mouse atria incubated with [3H]-noradrenaline. The receptors tested were facilitatory beta-adrenoceptors and angiotensin II receptors and inhibitory neuropeptide Y receptors. In order to evaluate the specificity of the N-ethylmaleimide treatment, we tested N-ethylmaleimide against the second messenger pathways that are linked to beta-adrenoceptors (adenylate cyclase) and angiotensin II (protein kinase C). The results show that a 60-min preincubation with N-ethylmaleimide (3 microM) abolished the facilitatory effect of isoprenaline (0.1 microM) and angiotensin II (0.1 microM) on the stimulation-induced release of noradrenaline and reduced the inhibitory action of neuropeptide Y (0.3 microM). N-ethylmaleimide had no effect on the stimulatory action of either phorbol dibutyrate (0.01, 0.1 microM), forskolin (10 microM), or a combination of 8-bromo adenosine-3'5'-monophosphate (90 microM) and 3-isobutyl-1-methylxanthine (100 microM). However, at a higher concentration (10 microM), N-ethylmaleimide reduced the facilitatory effect of phorbol dibutyrate (0.1 microM) and the combination of 8-bromo adenosine-3',5'-monophosphate (90 microM) and 3-isobutyl-1-methylxanthine (100 microM). This suggests that N-ethylmaleimide at 3 microM but not 10 microM was selective for receptor-mediated modulation of noradrenaline release without directly affecting the adenylate cyclase (forskolin, 8-bromo adenosine-3',5'-monophosphate + 3-isobutyl-1-methylxanthine) or protein kinase C (phorbol dibutyrate) transduction pathways. In atria from mice pretreated with pertussis toxin (1.5 micrograms/mouse), N-ethylmaleimide preincubation (1 and 3 microM) resulted in a more pronounced reduction of the inhibitory action of neuropeptide Y (0.3 microM). The nature of this interaction is unclear. Since N-ethylmaleimide has been shown in other studies to inactivate G-proteins, the inhibitory effect of N-ethylmaleimide on prejunctional beta-adrenoceptors, angiotensin II receptors and neuropeptide Y receptors of sympathetic nerves may suggest that G-proteins are involved with these receptors, although other effects of N-ethylmaleimide on the receptor coupling processes cannot be ruled out. Moreover, it appears that the concentration of N-ethylmaleimide used is critical since a higher concentration (10 microM) resulted in non-specific effects on signal transduction mechanisms in the present experimental conditions.

Involvement of cyclic nucleotides in prejunctional modulation of noradrenaline release in mouse atria

1 In mouse isolated atria previously incubated with [3H]-noradrenaline, 8-bromo-cyclic AMP (3-270 microM) produced a concentration-dependent increase in the fractional stimulation-induced outflow of radioactivity. 8-Bromo-cyclic GMP induced a lesser increase in the stimulation-induced outflow. 2 The phosphodiesterase inhibitors: M&B 22948 (90 microM); ICI 63197 (30 and 90 microM) and 3-isobutyl-1-methylxanthine (90 microM) increased the fractional stimulation-induced outflow. Together these results indicate that cyclic AMP may have a modulatory effect on noradrenaline release. 3 The inhibition of the stimulation-induced outflow produced by clonidine (0.03 microM) and its facilitation produced by phentolamine (1 microM) were unaltered in the presence of 8-bromo-cyclic AMP (90 microM). However, in the presence of 8-bromo-cyclic AMP (270 microM), the facilitatory effect of phentolamine was enhanced, but the inhibitory effect of clonidine (0.03 microM) was unaltered. In the presence of ICI 63197 (30 microM) the inhibitory effect of clonidine (0.03 microM) was unaltered, but the facilitatory effect of phentolamine (1 microM) was slightly enhanced. 4 Isoprenaline (0.003-0.1 microM) enhanced the fractional stimulation-induced outflow, an effect blocked by propranolol (0.1 microM). In the presence of 8-bromo-cyclic AMP (90 microM), the facilitatory effect of isoprenaline (0.01 microM) was blocked. In the presence of ICI 63197 (30 microM) the facilitatory effect of isoprenaline (0.003 microM) was potentiated. 5 These results suggest that whereas beta-adrenoceptor-mediated enhancement of noradrenaline release is linked to the stimulation of adenylate cyclase and enhanced formation of cyclic AMP, alpha-adrenoceptor-mediated inhibition of noradrenaline release is not linked to inhibition of adenylate cyclase activity.

Adrenergic receptors mediating prostaglandin production in the rabbit vas deferens

Contractile and prostaglandin E (PGE)-producing effects of adrenergic agonists were compared in the rabbit isolated vas deferens to determine which adrenergic receptor(s) potentially could mediate neural responses. Additionally, interactions among receptors were elucidated by comparing responses to norepinephrine, phenylephrine and isoproterenol to those in the presence of selective adrenergic agonists or antagonists. Norepinephrine increased the force of muscle contraction and the immunoassayable PGE concentrations in a concentration-dependent manner with EC50's of 55 +/- 8 and 112 +/- 39 microM, respectively. Propranolol (10 microM) enhanced the contractile effects of norepinephrine (p less than 0.01) whereas yohimbine (100 microM) or prazosin (1 microM) reduced norepinephrine-induced contractions and PGE production (p less than 0.01). Propranolol did not alter the PGE production induced by norepinephrine. Metoprolol (100 microM) also enhanced contractile effects of norepinephrine (p less than 0.05). The beta adrenergic agonist, isoproterenol (100 nM), decreased the contractile, but not the PGE-producing, effects of phenylephrine (p less than 0.001). Isoproterenol, given alone, increased PGE concentrations and inhibited electrically-induced force generation in a concentration-dependent manner. These results are consistent with the presence of alpha receptors on the vas deferens which mediate smooth muscle contraction and PGE generation. Beta receptors which mediate relaxation and PGE production also are present. Tentative identification of the beta receptor subtype revealed the presence of a beta 1 receptor.

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Prejunctional beta-adrenoceptors in rabbit pulmonary artery and mouse atria: effect of alpha-adrenoceptor blockade and phosphodiesterase inhibition

In rabbit isolated pulmonary artery previously incubated with [3H]-noradrenaline, isoprenaline (0.3 microM) had no effect on the stimulation-induced outflow of radioactivity. However, if the phosphodiesterase inhibitor ICI 63,197 (30 microM) or the alpha-adrenoceptor blocker phentolamine (1 microM) was present, then isoprenaline significantly enhanced the stimulation-induced outflow, an effect blocked by propranolol (0.1 microM). ICI 63,197 (30 microM) but not phentolamine significantly enhanced the stimulation-induced outflow of radioactivity. In mouse isolated atria previously incubated with [3H]-noradrenaline and stimulated at a frequency of 10 Hz, isoprenaline had no effect on the stimulation-induced outflow of radioactivity; this is in contrast to its release-enhancing effects at stimulation frequencies of 4 Hz and 2 Hz. The facilitation of stimulation-induced outflow by isoprenaline at 4 Hz was blocked by propranolol (0.08 microM) which, by itself, had no effect on the stimulation-induced outflow. At a stimulation frequency of 2 Hz in mouse atria the facilitatory effect of isoprenaline (0.01 microM) was significantly greater in the presence of ICI 63,197 (30 microM) which, by itself, had no effect on the stimulation-induced outflow. Similarly, the facilitatory effect of isoprenaline was significantly greater in the presence of phentolamine (1 microM) but, in this case, phentolamine significantly enhanced the stimulation-induced outflow. These results suggest that facilitatory prejunctional beta-adrenoceptors are present in both rabbit pulmonary artery and mouse atria. The effects of the phosphodiesterase inhibitor ICI 63,197 suggest that they are linked to adenylate cyclase in both tissues and we propose that the ability of phentolamine to facilitate the release and enhance the effect of isoprenaline may be due to the blockade of alpha-adrenoceptor inhibition of adenylate cyclase. This latter proposition needs further investigation.

Role of cAMP for regulation of impulse-evoked noradrenaline release from the rabbit pulmonary artery and its possible relationship to presynaptic ACTH receptors

Strips of the rabbit pulmonary artery preincubated with 3H-noradrenaline were superfused with physiological salt solution containing cocaine, corticosterone and propranolol. Basal tritium efflux and electrically evoked tritium overflow were determined. The basal efflux of tritium was not affected by forskolin 0.01-10 mumol/l, 8-Br-cAMP and dibutyryl-cAMP 10-330 mumol/l, or the phosphodiesterase inhibitors rolipram 1-10 mumol/l and AH 21-132 l mumol/l; it was increased by AH 21-132 10-100 mumol/l. Forskolin concentration-dependently increased the evoked 3H overflow, and this effect was not attenuated by omission of cocaine. The facilitatory effect of forskolin was more pronounced at 0.66 Hz than at 2 Hz. Rolipram, AH 21-132, 8-Br-cAMP or dibutyryl-cAMP also produced a concentration-dependent increase in evoked 3H overflow (8-Br-cAMP was more effective than dibutyryl-cAMP in this respect). Except for the highest concentration investigated, AH 21-132 was more effective in facilitating evoked overflow than in increasing basal efflux. Forskolin, AH 21-132 or 8-Br-cAMP did not alter the percentages of 3H-noradrenaline and 3H-metabolites contained in basal tritium efflux or in stimulation-evoked tritium overflow. When a combination of AH 21-132 plus 8-Br-cAMP or AH 21-132 plus forskolin was administered, the facilitatory effect on evoked tritium overflow was more pronounced than with the single compounds alone. ACTH1-24 also facilitated the evoked tritium overflow. Combined exposure to ACTH1-24 plus forskolin, ACTH1-24 plus AH 21-132 or ACTH1-24 plus forskolin plus AH 21-132 resulted in a clearly more pronounced increase in evoked tritium overflow than exposure to the single compounds alone.(ABSTRACT TRUNCATED AT 250 WORDS)