Role of calcium regulation in vascular hyporeactivity induced by peritonitis-associated septic shock

The norepinephrine (NE)-induced contraction contains two components: Ca²⁺ release from the sarcoplasmic reticulum (SR) as the fast (F) phase, and Ca²⁺ influx via a voltage-dependent calcium channel on the membrane as the slow (S) phase. This study was to evaluate the role of Ca²⁺ handling in vascular hyporeactivity to NE using functional isometric tension recording experiments in isolated septic rat aorta and mesenteric artery. The sepsis was induced by cercal ligation and puncture (CLP) and the vascular tissues were removed at the late phase of sepsis (i.e. 18 hours after CLP). Our results showed that rats which received CLP for 18 hours manifested severe hypotension and vascular hyporeactivity to NE in vivo. In addition, in-vitro studies showed that the vascular hyporeactivity to NE was not only observed in the aorta but also in the mesenteric artery obtained from the CLP-induced sepsis rat, and is more severe in the small resistance artery. Both the F phase and S phase of NE-induced contraction were reduced in aortas and mesenteric arteries from sepsis rats. The addition of 2.5 mM Ca²⁺ into Ca²⁺-free solution restored the NE-induced contraction in aortas to the level of that in normal Krebs' solution in both groups, while a delayed contraction occurred in the mesenteric artery from the CLP group. In order to clarify what possible mechanisms contribute to the abnormal calcium handling in sepsis, inhibitors of calcium channel and release were used. The inhibition of 2-aminoethoxy-diphenyl borate (2-APB), ryanodine, and cyclopiazonic acid (CPA) on the NE-induced contraction in Ca²⁺-free solution were greater in the aorta from septic rats, and inhibitions of CPA and ryanodine, but not of 2-APB, were attenuated by nitric oxide (NO) synthase inhibitor N^ω nitro-L-arginine methyl ester. In addition, the attenuation of NE-induced contraction by nifedipine in the aorta was also greater in the CLP group. Our results therefore suggest that vascular hyporeactivity to NE in the CLP-induced sepsis is caused by a major decrease of SR function and a minor impairment of voltage-dependent Ca²⁺ channels on membrane to Ca²⁺ handling in aortas and mesenteric arteries of rats, and this could be attributed to the overproduction of NO in sepsis.