Pharmacological assessment of Ca2+ dependence of endothelin-1-induced response in rat aorta

Mechanisms underlying the contractile response to endothelin-1 in the rat renal artery

We assessed the functional response and the mechanisms following receptor stimulation of endothelin-1 (ET-1) in the rat renal artery. In this study, isometric tension was recorded in renal artery rings without endothelium. Cumulative application of ET-1 from 0.1 to 100 nmol/l induced a sustained concentration-dependent contraction in the renal artery. Submaximal contraction induced by 10 nmol/l ET-1 in 2.5 mmol/l Ca(2+) and in the absence of inhibitors was used as control response (100%). The relative contribution of different sources of Ca(2+) in ET-1-induced contraction was evaluated. The contractile response to 10 nmol/l ET-1 in 2.5 mmol/l Ca(2+ )(1.2 +/- 0.2 g) was significantly inhibited either in Ca(2+)-free solution containing 100 micromol/l ethylene glycol bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (0.6 +/- 0.1 g) or after depletion of intracellular Ca(2+) stores (0.62 +/- 0.05 g). The contribution of phospholipase C and protein kinase C was evaluated by using their inhibitors 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate (NCDC) and [1-(5-isoquinolinesulfonyl)-2-methylpiperazine] (H-7), respectively. The contractile response to 10 nmol/l ET-1 was inhibited by 10 micromol/l NCDC (to 80 +/- 6%) and 30 micromol/l H-7 (to 76.6 +/- 6.5%). We found that 1 micromol/l nifedipine inhibited the ET-1-induced contraction (to 48.7 +/- 6.9%), indicating the contribution of Ca(2+) influx through voltage-gated L-type Ca(2+) channels to this response. Further, the inhibitory effect of nifedipine was to a greater extent as compared with NCDC or H-7. Additive inhibition of ET-1-induced contraction was not observed in the presence of both nifedipine and NCDC. We also evaluated the role of the ionic transport system in the ET-1-induced response by using 20 nmol/l 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), an inhibitor of Na(+)-H(+) exchange, or 100 micromol/l ouabain, an inhibitor of Na(+)-K(+)-ATPase. The response to ET-1 was decreased by both EIPA (to 61.6 +/- 8.4%) and ouabain (to 62.1 +/- 8.6%). The contribution of Na(+)-Ca(2+) exchange to ouabain action was tested using the inhibitor dimethyl amiloride HCl (10 micromol/l). The decrease in ET-1-induced contraction by the combination of ouabain and dimethyl amiloride HCl was similar to that observed with ouabain alone. In view of these observations, both extra- and intracellular sources of Ca(2+) contribute to the contractile response induced by ET-1 in the renal artery. Our findings also revealed the importance of Ca(2+) influx through voltage-gated L-type Ca(2+) channels in mediating contraction to ET-1 in the renal artery, whereas a minor role of phospholipase C and protein kinase C was observed. Na(+)-H(+) exchange and Na(+)-K(+)-ATPase also play a role in the ET-1-induced contraction in renal artery. Moreover, the contribution of Na(+)-K(+)-ATPase in ET-1 contraction is not an Na(+)-Ca(2+) exchange-related process.

Role of the purinergic and noradrenergic components in the potentiation by endothelin-1 of the sympathetic contraction of the rabbit central ear artery during cooling

1. To examine the role of the purinergic and noradrenergic components in the potentiation of endothelin-1 on the vascular response to sympathetic nerve stimulation, we recorded the isometric response of isolated segments, 2 mm long, from the rabbit central ear artery to electrical field stimulation (1-8 Hz) under different conditions, at 37 degrees C during cooling (30 degrees C). 2. Electrical field stimulation produced frequency-dependent contraction, which was reduced during cooling (about 60% for 8 Hz). Both at 37 degrees C and 30 degrees C, phentolamine (1 microM) or blockade of alpha(1)-adrenoceptors with prazosin (1 microM) reduced, whereas blockade of alpha(2)-adrenoceptors with yohimbine (1 microM) increased, the contraction to electrical field stimulation. This contraction was increased after desensitization of P2-receptors with alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-meATP, 3 microM) at 37 degrees C but not at 30 degrees C, and was not modified by blockade of P2-receptors with pyridoxalphosphate-6-azophenyl-2,4'-disulphonic acid (PPADS, 30 microM) at either temperature. 3. Endothelin-1 (1, 3 and 10 nM) at 37 degrees C did not affect, but at 30 degrees C it potentiated in a concentration-dependent manner the contraction to electrical field stimulation (from 28 +/- 6 to 134 +/- 22%, for 8 Hz). At 37 degrees C, endothelin-1 in the presence of phentolamine or prazosin, but not in that of yohimbine, alpha, beta-meATP or PPADS, potentiated the contraction to electrical stimulation. At 30 degrees C, phentolamine or yohimbine reduced, prazosin did not modify and alpha, beta-meATP slightly increased the potentiation by endothelin-1 of the response to electrical stimulation. 4. The arterial contraction to ATP (2 mM) and the alpha(2)-adrenoceptor agonist BHT-920 (10 microM), but not that to (-)-noradrenaline (1 microM), was potentiated by endothelin-1 at both 37 degrees C and 30 degrees C. 5. These results in the rabbit central ear artery suggest that the sympathetic response: (a) at 37 degrees C, could be mediated mainly by activation of alpha(1)-adrenoceptors, with low participation of P2-receptors, (b) is diminished during cooling, probably by a reduction in the participation of alpha(1)-adrenoceptors, and in this condition the response could be mediated in part by P2-receptors, and (c) is potentiated by endothelin-1 during cooling, probably by increasing the response of both postjunctional alpha(2)-adrenoceptors and P2-receptors.

Role of endothelin receptors, calcium and nitric oxide in the potentiation by endothelin-1 of the sympathetic contraction of rabbit ear artery during cooling

1. To examine further the potentiation by endothelin-1 on the vascular response to sympathetic stimulation, we studied the isometric response of isolated segments, 2 mm long, from the rabbit central ear artery to electrical field stimulation (1-8 Hz), under different conditions, at 37 degrees C and during cooling (30 degrees C). 2. Electrical stimulation produced frequency-dependent contraction, which was reduced (about 63% for 8 Hz) during cooling. At 30 degrees C, but not at 37 degrees C, endothelin-1 (1, 3 and 10 nM) potentiated the contraction to electrical stimulation in a dose-dependent way (from 43 +/- 7% to 190 +/- 25% for 8 Hz). 3. This potentiation by endothelin-1 was reduced by the antagonist for endothelin ETA receptors BQ-123 (10 microM) but not by the antagonist for endothelin ETB receptors BQ-788 (10 microM). The agonist for endothelin ETB receptors IRL-1620 (0.1 microM) did not modify the contraction to electrical stimulation. 4. The blocker of L-type Ca2+ channels verapamil (10 microM l-1) reduced (about 72% for 8 Hz) and the unspecific blocker of Ca(2+)-channels NiCl2 (1 mM) practically abolished (about 98%), the potentiating effects of endothelin-1 found at 30 degrees C. 5. Inhibition of nitric oxide synthesis with NG-nitro-L-arginine (L-NOARG, 0.1 mM) increased the contraction to electrical stimulation at 30 degrees C more than at 37 degrees C (for 8 Hz, this increment was 297 +/- 118% at 30 degrees C, and 66 +/- 15% at 37 degrees C). Endothelium removal increased the contraction to electrical stimulation at 30 degrees C (about 91% for 8 Hz) but not at 37 degrees C. Both L-NOARG and endothelium removal abolished the potentiating effects of endothelin-1 on the response to electrical stimulation found at 30 degrees C. 6. These results in the rabbit ear artery suggest that during cooling, endothelin-1 potentiates the contraction to sympathetic stimulation, which could be mediated at least in part by increasing Ca2+ entry after activation of endothelin ETA receptors. This potentiating effect of endothelin-1 may require the presence of an inhibitory tone due to endothelial nitric oxide.

Role of Ca2+ in the vascular contraction caused by a thrombin receptor activating peptide

Thrombin receptor activating peptide (TRAP) caused a slowly developing, sustained contraction of endothelium denuded rat aortic rings. Both nifedipine (10 microM) and removal of Ca2+ from the physiological salt solution (PSS) caused significant (60-75%) reductions in the contractile response to TRAP. In Ca(2+)-free PSS the response to both phenylephrine and TRAP were markedly reduced. Readministration of Ca2+ quickly restored the full response to phenylephrine. In contrast, readministration of Ca2+ only partially restored the TRAP response. Depletion of TRAP-sensitive intracellular Ca2+ stores had no effect on the phenylephrine response in Ca(2+)-free PSS. A threshold contracting concentration of TRAP (10 microM) enhanced contractions to the activator of voltage regulated Ca2+ channels Bay K 8644. Similarly, Bay K 8644 enhanced responses to TRAP. It is concluded that the contractile response of rat aortic rings to TRAP is largely mediated by influx of extracellular Ca2+. Furthermore, the intracellular Ca2+ pool(s) activated appears to be different from the phenylephrine-sensitive pools, which cannot be depleted by TRAP.

Inhibitory effects of nordihydroguaiaretic acid on ETA-receptor-mediated contractions to endothelin-1 in rat trachea

1. It has been shown previously that nordihydroguaiaretic acid (NDGA) inhibits endothelin-1 (ET-1)-induced contractions in rat isolated tracheal smooth muscle. To investigate the underlying mechanisms, this study examined the effects of NDGA on various aspects of the ETA and ETB receptor-effector systems which mediate ET-1-induced contractions in this preparation. 2. NDGA inhibited contractions induced by each of the isoforms of ET (ET-1, ET-2 and ET-3) but not those induced by the ETB receptor-selective agonist, sarafotoxin S6c, the cholinoceptor agonist, carbachol or the depolarizing spasmogen, KCl. 3. Quantitative autoradiographic studies of [125I]-ET-1 binding to rat tracheal smooth muscle indicated that NDGA was not an ET receptor antagonist. 4. NDGA inhibited the ETA receptor-mediated, intracellular Ca(2+)-dependent contractions induced by 100 nM ET-1 in Ca(2+)-free solution (by 75%, P < 0.01). Furthermore, NDGA markedly inhibited the contractions induced by ryanodine and cyclopiazonic acid; contractions purportedly due to Ca2+ release from intracellular stores. 5. Like NDGA, the sarcoplasmic reticulum Ca(2+)-ATPase inhibitors cyclopiazonic acid and thapsigargin inhibited contractions to ET-1, but not carbachol or KCl. However, cyclopiazonic acid, but not NDGA, also (a) induced transient contractions in rat trachea, (b) potentiated contractions induced by KCl, and (c) potentiated the extracellular Ca(2+)-dependent phase of ET-1-induced contractions, indicating that NDGA did not inhibit ET-1-induced contractions through Ca(2+)-ATPase inhibition and depletion of sarcoplasmic reticular Ca2+. 6. In control preparations, ET-1 induced a slowly developing, sustained contraction. However, in the presence of NDGA or the ETA receptor antagonist, BQ123, ET-1-induced contractions resembled the transient contractions induced by sarafotoxin S6c. In nominally Ca2+-free solution, ETA receptor mediated contractions induced by ET-1 developed very slowly and were inhibited by NDGA.7. Additional studies indicated that the inhibitory effects of NDGA on endothelin-1-induced contractions were not the result of any significant actions of NDGA on lipoxygenase, cytochrome P450, L- orT-type Ca2+-channels, Na+-channels or protein kinase C.8. In summary, NDGA selectively inhibited ET-1-induced contractions in rat tracheal smooth muscle via a lipoxygenase-independent mechanism involving inhibition of the ETA but not the ETB, receptor effector system. NDGA did not appear to inhibit the initial events in the ETA signal transduction pathway, such as receptor binding and protein kinase C activation. However, NDGA inhibited the intracellular Ca2+-dependent component of ET-1-induced contraction, possibly by inhibiting mobilisation of intracellular Ca2+. As an apparent direct consequence of inhibiting the ETA receptor-effector system, NDGA markedly changed the time course of ET-1-induced contractions; from a slowly developing and sustained contraction into a transient contraction resembling that induced by sarafotoxin S6c.

Endothelin-1 (ET-1)-induced contraction in rat isolated trachea: involvement of ETA and ETB receptors and multiple signal transduction systems

1. Quantitative autoradiographic, biochemical and functional studies were performed to investigate the endothelin receptor subtypes and signal transduction systems that mediate endothelin-1 (ET-1)-induced contraction in rat isolated tracheal smooth muscle. 2. Specific binding of 0.5 nM [125I]-ET-1 to tracheal smooth muscle was inhibited by at least 40% in the presence of either the ETA receptor selective ligand BQ-123 (1 microM) or the ETB receptor-selective ligand sarafotoxin S6c (30 nM), indicating the presence of both ETA and ETB receptors in this tissue. 3. ET-1 and sarafotoxin S6c were both potent spasmogens of rat isolated tracheal smooth muscle preparations. Sarafotoxin S6c-induced contractions were unaffected in the presence of the ETA receptor antagonist BQ-123 (10 microM), but were markedly attenuated in tissue previously exposed to 100 nM sarafotoxin S6c to induce ETB receptor desensitization. ET-1-induced contractions were, at most, only partially attenuated either by blocking the ETA receptor-effector system (with 10 microM BQ-123) or by desensitizing the ETB receptor-effector system with sarafotoxin S6c. However, ET-1-induced contractions were markedly attenuated by blocking both receptor-effector systems simultaneously. These findings suggest that ET-1 could induce contraction by stimulating either ETA or ETB receptors. 4. ET-1 (10 microM) induced a 7 fold increase in intracellular [3H]-inositol phosphate accumulation over basal levels in rat isolated tracheal smooth muscle. In contrast, sarafotoxin S6c (2.5 microM) increased intracellular [3H]-inositol phosphate accumulation by only 2 fold. ET-1-induced accumulation of [3H]-inositol phosphates was abolished by 10 microM BQ-123. 5. In Ca2+-free Krebs bicarbonate solution, 100 nM ET-1 induced a significantly larger contraction than that induced by 100 nM sarafotoxin S6c (46.6 +/- 5.6% C,., versus 8.8 +/- 2.8% Cmax, n = 5-7). This presumed intracellular Ca2+-dependent phase of contraction induced by ET-1 was significantly inhibited by 10 microM BQ-123 (7.5 +/- 1.0% C.). Subsequent addition of 2.5 mM Ca2+ induced a second phase of contraction. The extracellular Ca2+-dependent phase of contraction induced by ET-1 was similar inmagnitude to that induced by sarafotoxin S6c (63.6 +/- 4.5% C.. versus 58.0 +/- 3.7% C.) and was not inhibited by BQ-123. Sarafotoxin S6c-induced contractions were not inhibited by the L-type Ca2+-channel antagonists, nicardipine or verapamil.6. In summary, ETA and ETB receptors coexist in rat isolated tracheal smooth muscle and stimulation of both receptor subtypes contributes to ET-l-induced contraction in this tissue. However, stimulation of these receptor subtypes appears to induce contraction by activating different second messenger pathways; ETA receptor stimulation induces phosphoinositide turnover and subsequent release of intracellular Ca2+ whereas stimulation of ETB receptors facilitates the influx of extracellular Ca2+.