HEME OXYGENASE INHIBITOR ZINC PROTOPORPHYRIN-IX CAUSES AND ACTIVATIONOF NITRIC-OXIDE SYNTHASE IN THE RABBIT INTERNAL ANAL-SPHINCTER

The studies were performed in the rabbit internal anal sphincter (IAS) smooth muscle strips to examine the influence of the heme oxygenase i nhibitor, [zinc protporphyrin (ZnPP IX)], on basal tone. ZnPP IX pradu ced a concentration-dependent fall in the basal tone and was the focus of our investigation, To examine the mechanism of the fall in IAS ton e by ZnPP IX, the effect of different concentrations of ZnPP IX on bas al IAS tone and the release of nitric oxide were examined before and a fter the neurotoxin tetrodotoxin and various nitric oxide synthase (NO S) inhibitors, The inhibitory effect of ZnPP IX was blocked by the NOS inhibitors L-N-G-nitroarginine, N-G-monomethyl-L-arginine and L-N-5-( 1-iminoethyl)omithine and the neurotoxin TTX. The fall in IAS tension by ZnPP IX was accompanied by a release of NO. ZnPP IX (1 x 10(-3) M) caused a fall in IAS tension of 43.7% which was reduced to 16.5% in th e presence of L-N-G-nitroarginine (1 x 10(-4) M). Furthermore, the fai l in IAS tone in the presence of ZnPP IX was restored both by the NOS inhibitors and tetrodotoxin. The basal release of nitric oxide in thes e experiments was 0.50 +/- 0.07 nmol and in the presence of Zn PP IX ( 1 x 10(-3) M), it increased to 1.72 +/- 0.28 nmol (more than a 3-fold increase). Thus the fall in the basal IAS tone by ZnPP IX was associat ed with a release of NO from the myenteric neurons as these effects we re significantly blocked by the NOS inhibitors and tetrodotoxin. We co nclude that in the rabbit IAS, ZnPP IX causes a fall in the basal IAS tension by the activation oi NOS in myenteric neurons. We speculate th at in the resting state, the heme oxygenase pathway exerts important c ounter-regulatory effects on the NOS pathway and when blocked (e.g., b y ZnPP IX), the underlying NOS pathway is unmasked leading to a massiv e and prolonged release of NO. The exact significance of this mechanis m remains to be determined.