One of the key signal transduction steps activated by Et1 in vascular smooth muscle is an increase in intracellular [Ca
2+] ([Ca
2+]
i). Early studies described an initial Et-induced Ca
2+-transient that was dependent on phospholipase C activity and inositol 1,4,5 trisphosphate (IP
3) production. This was followed by a sustained rise in Ca
2+ because of the influx of extracellular Ca
2+.
15 –17 Use of microfluorometry to record average changes in intracellular [Ca
2+] ([Ca
2+]
i) in the smooth muscle layer of intact segments of rat retinal arterioles demonstrated that activation of the Et
ARs by Et1 resulted in both transient and sustained [Ca
2+]
i increases, stimulating vasoconstriction.
18 However, recent studies from our laboratory using high-speed Ca
2+-imaging have revealed faster cellular and subcellular Ca
2+-signaling events in retinal arteriolar myocytes that were not apparent in microfluorometry records from arteriole segments. These are seen in both rat and pig arterioles and consist of brief localized Ca
2+-sparks and more global Ca
2+-waves and oscillations, the latter associated with cell contraction.
19 –21 Studies in other vascular smooth muscle have shown that, at the cellular level, many vascular agonists act to increase the frequency of phasic [Ca
2+]-signals rather than uniformly raising mean [Ca
2+]
i.
22 –24 The experiments described here were designed to investigate Et1-evoked Ca
2+-signaling with high spatial and temporal resolution in retinal arteriolar myocytes for the first time and to examine the mechanisms responsible for the effects seen. They revealed dramatic Et1-induced increases in Ca
2+-sparks and oscillations, suggesting that regulation of constriction by Et1 relies more on frequency-modulated than amplitude-modulated signaling at the cell level.