Mechanisms underlying spontaneous rhythmical contractions in irideal arterioles of the rat

1. Mechanisms underlying spontaneous rhythmical contractions hare been stud ied in irideal arterioles of the rat using video microscopy and electrophys iology. 2. Rhythmical contractions (4 min(-1)) were more common during the second a nd third postnatal weeks and were always preceded by large, slow depolariza tions (5-40 mV). 3. Spontaneous contractions were unaffected by tetrodotoxin (1 mu M), neuro transmitter receptor antagonists, the sympathetic neurone blocker, guanethi dine (5 mu M) or sensory neurotoxin, capsaicin (1 mu M). 4. Stimulation of sensory nerves inhibited spontaneous activity and this wa s not prevented by L-NAME (10 mu M). 5. L-NAME (10 mu M) caused an increase in frequency of spontaneous contract ions, while forskolin (30 nM), in the presence of L-NAME, abolished spontan eous, but not nerve-mediated, contractions. 6. Spontaneous activity was not affected by felodipine (1 nM) or nifedipine (1 mu M), but was abolished by cadmium chloride (1 mu M) or superfusion wi th calcium-free solution. 7. Caffeine (1 mM), thapsigargin (2 mu M) and cyclopiazonic acid (3 mu M), but not ryanodine (3 mu M), abolished spontaneous and nerve-mediated contra ctions. After preincubation in L-NAME (10 mu M), cyclopiazonic acid abolish ed spontaneous contractions only. 8. Spontaneous depolarizations and contractions were abolished by 18 alpha- glycyrrhetinic acid (20 mu M) 9. Results suggest that spontaneous rhythmical contractions are myogenic an d result from the cyclical release of calcium from intracellular stores, wi thout a contribution from voltage-dependent calcium channels. Intercellular coupling through gap junctions appears to be essential for co-ordination o f these events which could be modulated by nitric oxide and increases in cA MP. The possibility that different intracellular stores underly spontaneous and nerve-mediated contractions is discussed.