Ca(2+)-sparks constitute elementary building blocks for global Ca(2+)-signals in myocytes of retinal arterioles

Spontaneous Ca(2+)-events were imaged in myocytes within intact retinal arterioles (diameter <40 μm) freshly isolated from rat eyes. Ca(2+)-sparks were often observed to spread across the width of these small cells, and could summate to produce prolonged Ca(2+)-oscillations and contraction. Application of cyclopiazonic acid (20 μM) transiently increased spark frequency and oscillation amplitude, but inhibited both sparks and oscillations within 60 s. Both ryanodine (100 μM) and tetracaine (100 μM) reduced the frequency of sparks and oscillations, while tetracaine also reduced oscillation amplitude. None of these interventions affected spark amplitude. Nifedipine, which blocks store filling independently of any action on L-type Ca(2+)-channels in these cells, reduced the frequency and amplitude of both sparks and oscillations. Removal of external [Ca(2+)] (1 mM EGTA) also reduced the frequency of sparks and oscillations but these reductions were slower in onset than those in the presence of tetracaine or cyclopiazonic acid. Cyclopiazonic acid, nifedipine and low external [Ca(2+)] all reduced SR loading, as indicated by the amplitude of caffeine evoked Ca(2+)-transients. This study demonstrates for the first time that spontaneous Ca(2+)-events in small arterioles of the eye result from activation of ryanodine receptors in the SR and suggests that this activation is not tightly coupled to Ca(2+)-influx. The data also supports a model in which Ca(2+)-sparks act as building blocks for more prolonged, global Ca(2+)-signals.