Previously we reported correlations between cell size and properties of the spontaneous intracellular calcium transient for single cells isolated from the rabbit sinoatrial node (SAN) (Lancaster et al. 2001). The aim of the present study was to investigate the mechanisms responsible for these. Single SAN cells were isolated from New Zealand White rabbits humanely killed by intravenous overdose of anaesthetic. At 37 °C cells contracted spontaneously and intracellular calcium transients were observed using the fluorescent indicator fluo-3. Cell size was determined from the projected video image of the cell, in mm². Sarcoplasmic reticulum (SR) calcium content was released by rapid application of 20 mM caffeine. For analysis data were divided about the mean cell area (611 ± 28 mm² (mean ± S.E.M.)) and classed as small or large cells for comparison using Student’s t test (unpaired). Significant correlations, as assessed using a linear regression, were found between the amplitude of the spontaneous calcium transient and cell size (P < 0.0001; n = 39) and the amplitude of the caffeine-evoked transient and cell size (P < 0.0001; n = 39). Large cells had a larger spontaneous and caffeine transient amplitude (P < 0.01) than small cells. The spontaneous calcium transient expressed as a fraction of the caffeine transient significantly correlated with cell size (P = 0.03; n = 39) large cells having a smaller fractional release of the total SR content with each spontaneous transient (P < 0.001). The time constant for the decay of the caffeine transient correlated with cell size both in the presence and absence of nickel (P = 0.04 in both cases, n = 39 and n = 19, respectively), the transient decaying more slowly in small cells (P = 0.02). In some instances it was not possible to obtain reliable data for the decay of the caffeine response due to the rapid resumption of spontaneous activity in the maintained presence of caffeine. From the rate constants for the decay of the caffeine transient in the absence and presence of 10 mM nickel, used to block the sodium calcium exchanger, the relative contributions of the sarcolemmal calcium ATPase and sodium calcium exchanger to cytoplasmic calcium removal were calculated. Calcium ATPase removed a mean of 19 ± 0.6 %, sodium calcium exchanger 81 ± 0.6 %. Significant correlations were found between both the calcium ATPase (P < 0.001) and sodium calcium exchange activity (P = 0.006) and cell size. Smaller cells had a smaller calcium removal by the calcium ATPase (P = 0.02). Previous work (Bleeker et al. 1980) reported a correlation between cell size and location, smaller cells being located towards the centre of the SAN larger cells in the periphery. The results are therefore indicative of a low SR calcium content in cells from the centre of the SAN, lower sarcolemmal calcium transporter activity and a change in the relative contribution of the cellular sarcolemmal calcium transport systems from the periphery to the centre. The recruitment of the SR calcium content with each spontaneous depolarisation is greater in the central cells. These results add further intrigue to the problem of understanding the regulation of intracellular calcium within the mammalian SAN and the role it plays.

This work was supported by the British Heart Foundation.

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