Ca2+sparks promote myogenic tone in retinal arterioles

Background and PurposeCa2+imaging reveals subcellularCa2+sparks and globalCa2+waves/oscillations in vascular smooth muscle. It is well established thatCa2+sparks can relax arteries, but we have previously reported that sparks can summate to generateCa2+waves/oscillations in unpressurized retinal arterioles, leading to constriction. We have extended these studies to test the functional significance ofCa2+sparks in the generation of myogenic tone in pressurized arterioles.Experimental ApproachIsolated retinal arterioles (25–40 μm external diameter) were pressurized to 70 mmHg, leading to active constriction.Ca2+signals were imaged from arteriolar smooth muscle in the same vessels usingFluo4and confocal laser microscopy.Key ResultsTone development was associated with an increased frequency ofCa2+sparks and oscillations. Vasomotion was observed in 40% of arterioles and was associated with synchronization ofCa2+oscillations, quantifiable as an increased cross‐correlation coefficient. Inhibition ofCa2+sparks with ryanodine, tetracaine, cyclopiazonic acid or nimodipine, or following removal of extracellularCa2+, resulted in arteriolar relaxation. Cyclopiazonic acid‐induced dilatation was associated with decreasedCa2+sparks and oscillations but with a sustained rise in the mean global cytoplasmic[Ca2+]([Ca2+]c), as measured usingFura2and microfluorimetry.Conclusions and ImplicationsThis study provides direct evidence thatCa2+sparks can play an excitatory role in pressurized arterioles, promoting myogenic tone. This contrasts with the generally accepted model in which sparks promote relaxation of vascular smooth muscle. Changes in vessel tone in the presence of cyclopiazonic acid correlated more closely with changes in spark and oscillation frequency than global[Ca2+]c, underlining the importance of frequency‐modulated signalling in vascular smooth muscle.