The regulation of cardiac activity by nitric oxide (NO) in the Vietnamese stick insect, Baculum extradentatum

This study examines the role of the unconventional gaseous signaling molecule nitric oxide (NO) on the regulation of heart rate in the Vietnamese stick insect, Baculum extradentatum. Using nicotinamide dinucleotide hydrogen phosphate (NADPH)-diaphorase histochemistry, as well as immunohistochemistry and Western blotting with an antibody against NO synthetase (NOS), we identified the presence of NOS in hemocytes present throughout the lumen of the dorsal vessel. We propose that NO is delivered to heart muscle tissue via hemocytes circulating within the hemolymph. In the present study, stimulation of NO levels by the application of the NO donor MAHMA-NONOate and l-arginine led to a dose-dependent decrease in heart rate. Treatment of tissues with the NOS inhibitor, L-NAME, in equimolar concentrations with l-arginine, led to a recovery of heart rate, without modifying heart rate on its own. Finally guanosine 3',5'-cyclic monophosphate (cGMP) analog, 8-bromo-cGMP, elicited similar inhibitory effects on stick insect heart rate as did the guanylate cyclase activator, YC-1, and the phosphodiesterase inhibitor, dipyridamole, indicating that cGMP is most likely the second messenger in the stick insect NO signaling pathway. Contrary to the cardioexcitatory effect of NO on other insect hearts, we have found that NO inhibits stick insect heart rate independently from any nervous system input, in a similar inhibitory fashion as that of vertebrate hearts.