NOREPINEPHRINE‐STIMULATED BREAKDOWN OF TRIPHOSPHOINOSITIDE OF RABBIT IRIS SMOOTH MUSCLE: EFFECTS OF SURGICAL SYMPATHETIC DENERVATION AND IN VIVO ELECTRICAL STIMULATION OF THE SYMPATHETIC NERVE OF THE EYE

Abstract

Abstract— Paired iris smooth muscles from rabbits were prelabelled either in vitro by incubation for 30 min at 37°C in an iso‐osmotic salt medium containing glucose, inositol, cytidine and 32Pi, or in vivo by administration of the isotope intracamerally into each eye 1 h before death. One of the pair was then incubated at 37°C for 10 min in an unlabelled medium containing 10 mm of 2‐deoxyglucose and the other was incubated in the presence of norepinephrine (NE) or other adrenergic agents. Triphosphoinositide (TPI) was found to contain more 32P than any other phospholipid (almost 39% of total lipid radioactivity) in both the in vitro and in vivo experiments. NE (50 μm) increased the loss of 32P from TPI (the TPI effect') by 28–30% in the 32P‐labelled muscle. The TPI effect was accompanied by a significant increase in 32P labelling of phosphatidic acid (PA) and phosphatidylinositol (PI) but not phosphatidylcoholine. In this tissue the TPI effect was found to be mediated through α‐adrenergic receptors. At 14 days after surgical sympathetic denervation, incorporation of 32P into phospholipids of the denervated muscle increased by an average of 6% over that of the normal muscle. The increase in TPI, PI and PA was 7%, 4% and 9% of that of the control respectively. There was little change in phospholipid content of the denervated muscle. The increase in sensitivity to NE (12.5 μm) caused by denervation produced about 18% increase in the TPI effect and a 25% increase in the 32P labelling of PA, but not PI. In view of our previous findings on the requirement of the TPI effect for Ca2+, this observation could suggest that an increase in Ca2+ influx, following the interaction between the neurotransmitter and its receptor could stimulate TPI‐phosphodiesterase, thus leading to increased PA via increased diglyceride. This denervation‐induced supersensitivity to NE appears to be postsynaptic in nature. 32Pi was injected intracamerally into each eye 1 h before electrical stimulation of one of the sympathetic trunks. After stimulation for 30 min there was a significant loss of 32P from TPI and a significant increase in the labelling of PI and PA of the stimulated muscle. It is concluded that TPI and its enzymes could play an important role in neurotransmission at the neuromuscular junction of smooth muscle.