Absence of force suppression in rabbit bladder correlates with low expression of heat shock protein 20

Abstract

Abstract Background Nitroglycerin can induce relaxation of swine carotid artery without sustained reductions in [Ca2+]i or myosin regulatory light chain (MRLC) phosphorylation. This has been termed force suppression and been found to correlate with ser16-phosphorylation of heat shock protein 20 (HSP20). We tested for the existence of this mechanism in a smooth muscle that is not responsive to nitric oxide. Methods Isometrically mounted mucosa free rabbit bladder strips were contracted with carbachol and relaxed with 8-Br-cGMP, forskolin, or isoprenaline. Results Contraction was associated with a highly cooperative relation between MRLC phosphorylation and force such that very small increases in MRLC phosphorylation induced large increases in force. Relaxation induced by 8-Br-cGMP, forskolin, or isoprenaline did not shift the MRLC phosphorylation-force relation from that observed with carbachol alone, i.e. there was no force suppression. HSP20 content was negligible (approximately two hundred-fold less than swine carotid). Conclusion The lack of force suppression in the absence of HSP20 is consistent with the hypothesized role for HSP20 in the force suppression observed in tonic smooth muscles.