okadaic-acid and zaprinast

The aim of the present study was to characterize the influence of the phosphatase type 1 and 2A inhibitor okadaic acid on non-adrenergic, non-cholinergic (NANC) neurotransmission in the rat gastric fundus. Okadaic acid (10-6 M), an inhibitor of protein phosphatases 1 and 2A, did not show any influence on the basal tonus or on a contraction plateau induced by 5-HT (10-7 M) within 30 min of observation. When okadaic acid (10-6 M) was applied 10 min prior to 5-HT (10-7 M), the contraction plateau of serotonin was unchanged. To investigate the inhibitory neurotransmission, the muscle strips were pre-contracted using 5-HT (10-7 M), and inhibitory stimuli were applied at the contraction plateau, which was stable over 30 min. The inhibitory effects of vasoactive intestinal peptide (VIP), nitric oxide (NO) and electrical field stimulation (EFS, 40 V, 0.5 ms, frequencies ranging from 0.5 to 16 Hz) were examined. When okadaic acid (10-6 M) was applied prior to EFS-induced NANC relaxation, significant attenuation of the inhibitory response was demonstrated (16 Hz: control: -92.4 +/- 1.9%; okadaic acid 10-7 M: -60.7 +/- 6.1%; okadaic acid 10-6 M: -25.3 +/- 3.4%; n=11; P < 0.01). By contrast, neither the concentration-dependent inhibitory actions of VIP (10-11-10-8 M) (VIP 10-8 M: -100%; VIP 10-8 M + okadaic acid 10-6 M: -89.9 +/- 8.3%; n=8; n.s) nor that of diethylamine nitric oxide (DEA-NO) (3 x 10-7-10-4 M) (DEA-NO 10-4 M: -95.3 +/- 8.4%; DEA-NO 10-4 M + okadaic acid 10-7 M: -98.3 +/- 6.3%; DEA-NO 10-4 M + okadaic acid 10-6 M: 96.5 +/- 7.6%; n=9; n.s.) on 5-HT induced contraction were altered by pre-incubation with okadaic acid (10-6 M). This is the first report that supports the concept that protein phosphatases 1 and 2A may contribute to the regulation of rat gastric fundus motility. The protein phosphatase inhibitor okadaic acid significantly reduces electrically induced inhibitory NANC responses, while leaving direct muscular effects of the inhibitory NANC neurotransmitters VIP and NO unaffected - suggesting a neural site of action. The potential roles of protein phosphatases on NANC neurotransmission remain to be clarified in detail, as this might offer a new pathway for modulating smooth-muscle function.

Topics: Animals; Carbazoles; Electric Stimulation; Enzyme Inhibitors; Gastric Fundus; Gastrointestinal Agents; Hydrazines; In Vitro Techniques; Indoles; Male; Muscle Relaxation; Muscle, Smooth; Nitric Oxide; Nitric Oxide Donors; Nitroarginine; Nitrogen Oxides; Okadaic Acid; Oxadiazoles; Phosphodiesterase Inhibitors; Phosphoprotein Phosphatases; Purinones; Pyrroles; Quinoxalines; Rats; Rats, Wistar; Synaptic Transmission; Vasoactive Intestinal Peptide

1. Possible contribution of K+ channel opening to the relaxation by glyceryl trinitrate (GTN) was examined using isolated rabbit aorta. 2. While glibenclamide and apamin failed to affect relaxation by GTN, both charybdotoxin (ChTx) and iberiotoxin (IbTx) effectively attenuated GTN-induced relaxation. 3. The increase in cGMP produced by GTN was not attenuated by ChTx and IbTx. 4. The inhibitory effect of ChTx on GTN-induced relaxation was not reduced in the presence of zaprinast, indicating that cGMP but not GMP was responsible for activation of the K+ channel. 5. Okadaic acid, a selective inhibitor of protein phosphatase 2A, had no effect on the relaxation by GTN. These results indicate that, though small in degree, activation of a ChTx-sensitive K+ channel (large conductance Ca(2+)-activated K+ channel) is involved in the GTN-induced relaxation in rabbit aorta.

Topics: 3',5'-Cyclic-GMP Phosphodiesterases; Animals; Aorta, Thoracic; Apamin; Charybdotoxin; Cyclic GMP; Endothelium, Vascular; Ethers, Cyclic; Glyburide; In Vitro Techniques; Male; Muscle Contraction; Muscle Relaxation; Muscle, Smooth, Vascular; Nitroglycerin; Okadaic Acid; Peptides; Phosphoprotein Phosphatases; Potassium Channels; Protein Phosphatase 2; Purinones; Rabbits; Scorpion Venoms