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The role of capacitative Ca 2 + influx in the α lB ‐adrenoceptor‐mediated contraction to phenylephrine of the rat spleen
Author(s) -
Burt Richard P.,
Chapple Christopher R.,
Marshall Ian
Publication year - 1995
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1995.tb15073.x
Subject(s) - phenylephrine , egta , contraction (grammar) , cyclopiazonic acid , chemistry , muscle contraction , endocrinology , medicine , biophysics , biochemistry , calcium , biology , endoplasmic reticulum , organic chemistry , blood pressure
1 The mechanism of contraction to phenylephrine in the rat spleen (mediated via α 1B‐ adrenoceptors) has been studied in functional experiments. 2 The concentration‐dependent contraction of the rat spleen to cumulative additions of phenylephrine (pD 2 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 Ca 2+ (2.5 mM) to a single concentration of phenylephrine (3 × 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 × 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 × 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 Ca 2+ (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 Ca 2+ (2.5 mM) to the Krebs solution a tonic contraction developed in the tissue (263 ± 12% maximum response to phenylephrine). 6 In Ca 2+ ‐free Krebs solution, after the spleen had been incubated with cyclopiazonic acid for 30 min, the subsequent contraction to phenylephrine (3 × 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 Ca 2+ channel blocker, SK&F 96365 (3times10 −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 × 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 × 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 Ca 2+ and a larger tonic contraction due to capacitative Ca 2+ influx through non‐voltage‐gated Ca 2+ channels and which may involve a tyrosine kinase. This suggests that inositol triphosphate but not diacylglycerol is involved in the contraction.