The role of capacitative Ca2+ influx in the αlB‐adrenoceptor‐mediated contraction to phenylephrine of the rat spleen
Authors: Burt Richard P. , Chapple Christopher R. , Marshall Ian 
Published: July 2012

The mechanism of contraction to phenylephrine in the rat spleen (mediated via α1B‐adrenoceptors) has been studied in functional experiments.

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).

Contraction of the rat spleen in normal Krebs solution containing Ca2+ (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 Ca2+‐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.

In some tissues after the contraction to phenylephrine (3 × 10−4 m) in Ca2+‐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).

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 Ca2+‐free Krebs solution containing EGTA (30+4% of the maximum response to phenylephrine). One hour after the end of the contraction in Ca2+‐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).

In Ca2+‐free Krebs solution, after the spleen had been incubated with cyclopiazonic acid for 30 min, the subsequent contraction to phenylephrine (3 × 10−4m) was reduced from 46 ± 3% to 9 ± 2% maximum response to phenylephrine.

Cumulative contractions to phenylephrine and the contraction to cyclopiazonic acid (10−5m) in the spleen were not significantly affected by nifedipine (10−6 m). The non‐selective Ca2+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.

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+.

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.