The role of capacitative Ca2+ influx in the alpha 1B-adrenoceptor-mediated contraction to phenylephrine of the rat spleen.
1. The mechanism of contraction to phenylephrine in the rat spleen (mediated via alpha 1B-adrenoceptors) has been studied in functional experiments. 2. The concentration-dependent contraction of the rat spleen to cumulative additions of phenylephrine (pD2 4.8 +/- 0.1) was not significantly reduced by the selective protein kinase C (PKC) inhibitor, calphostin C (10(-6)M) or potentiated by the DAG kinase inhibitor, R59022 (10(-6) M). 3. Contraction of the rat spleen in normal Krebs solution containing Ca2+ (2.5 mM) to a single concentration of phenylephrine (3 x 10(-4) M) produced a maximal response consisting of an initial phasic component and a more slowly developing tonic component. However in Ca(2+)-free Krebs solution (containing EGTA), phenylephrine (3 x 10(-4)M) produced only a phasic contraction which was reduced to 46 +/- 3% maximum response to phenylephrine in normal Krebs solution. 4. In some tissues after the contraction to phenylephrine (3 x 10(-4) M) in Ca(2+)-free Krebs solution (containing EGTA), the phenylephrine was washed out and the tissue was allowed to recover. After 2 h, upon addition of Ca2+ (2.5 mM) to the Krebs solution (EGTA now removed) a tonic contraction developed in the tissue (97 +/- 4% maximum response to phenylephrine). 5. Cyclopiazonic acid produced a tonic contraction of the rat spleen with a maximum effect at 10(-5) M (202 +/- 8% maximum response compared with that to phenylephrine). The contraction to CPA (10(-5) M) was reduced in Ca(2+)-free Krebs solution containing EGTA (30 +/- 4% of the maximum response to phenylephrine). One hour after the end of the contraction in Ca(2+)-free Krebs solution (EGTA now removed), upon addition of Ca2+ (2.5 mM) to the Krebs solution a tonic contraction developed in the tissue (263 +/- 12% maximum response to phenylephrine). 6 In Ca2+-free Krebs solution, after the spleen had been incubated with cyclopiazonic acid for 30 min,the subsequent contraction to phenylephrine (3 x 10-4 M) was reduced from 46+/-3% to 9+/-2%maximum response to phenylephrine.7 Cumulative contractions to phenylephrine and the contraction to cyclopiazonic acid (10-5 M) in the spleen were not significantly affected by nifedipine (10-6 M). The non-selective Ca2+channel blocker,SK&F 96365 (3 x 10-5 M) reduced the maximum response for the cumulative additions of phenylephrine to 35+/-1% and the contraction to CPA (10-5 M) from 202+/-8% to 108+/-8% maximum response to phenylephrine.8 The tyrosine kinase inhibitors genistein (3 x 10-5 M and tyrphostin 23 (10-4 M), reduced the maximum response to phenylephrine in the spleen to 51+/-4% and 44+/-5% respectively and the maximum contraction to cyclopiazonic acid (3 x 10-6 M) in the spleen from 132 +/- 6% to 82 +/-5% and 80 +/- 7% maximum response to phenylephrine respectively without affecting contractions to K+.9 In conclusion, these results are consistent with the contraction of the rat spleen to phenylephrine consisting of an initial phasic contraction due to release of intracellular Ca2+ and a larger tonic contraction due to capacitative Ca2+ influx through non-voltage-gated Ca2+ channels and which may involve a tyrosine kinase. This suggests that inositol triphosphate but not diacylglycerol is involved in the contraction.
Burt
R P
RP
Department of Pharmacology, University College London.
Chapple
C R
CR
Marshall
I
I
D058646:Adrenergic alpha-1 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D009281:Naphthalenes / Q000494:pharmacology; D009543:Nifedipine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors; D051381:Rats; D017207:Rats, Sprague-Dawley; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects*; D013154:Spleen / Q000187:drug effects*
D058646:Adrenergic alpha-1 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D002121:Calcium Channel Blockers; D004791:Enzyme Inhibitors; D007211:Indoles; D009281:Naphthalenes; D018340:Receptors, Adrenergic, alpha-1; D010656:Phenylephrine; D011505:Protein-Tyrosine Kinases; D011493:Protein Kinase C; C058819:calphostin C; D009543:Nifedipine; D002118:Calcium; C000543:cyclopiazonic acid