By Cx43 immuno-labeling of freshly isolated adult rabbit SANC, we confirmed that the average cell size (projected area) from the central area (Cx43-negative, 612.9 plus/minus 12.5 micrometers-squared, n=340) is indeed smaller than SANC from the peripheral area (Cx43-positive, 818.6 plus/minus 23.7 micrometers-squared, n=188, p<0.001), though the median values of the cell size are 571.0 and 747.5 micrometers-squared for Cx43-negative and Cx43-positive SANC, respectively, suggesting an uneven distribution for both cell originals. On top of that, the Cx43-negative SANC tend to have relatively longer perimeters, as their ratio of perimeter and projected area (0.350 plus/minus 0.005) is significantly bigger than the Cx43-positive SANC (0.300 plus/minus 0.006, p<0.001). Is there any functional relevance related to the cell origin? We did not detect any difference in AP firing rate (3.07 plus/minus 0.13 Hz, n=13 for Cx43-negative and 3.28 plus/minus 0.12 Hz, n=23 for Cx43-positive, respectively, p=0.26), though the cell size of Cx43-negative SANC (657.4 plus/minus 45.2 micrometers-squared) is smaller than Cx43-positive SANC (1020.0 plus/minus 61.0 micrometers-squared, p<0.001). In addition, there is no significant difference of AP amplitude, Maximum Diastolic Polarization or AP overshoot between Cx43-negative and Cx43-positive SANC, and no correlation was found between these amplitude-related parameters and cell size. On the other hand, we detected some substantial difference in AP characteristic parameters between Cx43-negative and Cx43-positive SANC. Cx43-negative SANC have a longer AP duration than Cx43-positive (APD50 = 93.0 plus/minus 4.0 vs. 78.0 plus/minus 2.9 ms, p<0.01;APD75 = 120.7 plus/minus 4.9 vs. 102.4 plus/minus 2.9, p<0.01), slower AP upstroke speed (dV/dtmax = 8.12 plus/minus 1.31 vs. 13.36 plus/minus 0.62 V/sec, p<0.001), but shorter linear diastolic depolarization (p<0.001). However, linear regression analysis failed to detect significant correlations between these kinetic parameters and cell size for both Cx43-negative and Cx43-positive SANC! Our preliminary data of AP triggered global Ca2+ transient do not show significant difference of the major characteristic parameters between Cx43-negative and Cx43-positive SANC, though the cell size is different. In the same set of cells, spontaneous Local Ca2+ Releases (LCRs) are present beneath the cell membrane just prior to AP-induced global Ca2+ transient. No substantial difference is found in terms of LCR period, amplitude, size and duration between the two groups. The total LCR signal mass per cell is similar (p=0.85) in both cell originals, but it is compromised at fewer LCRs (p<0.05) but larger signal mass per LCR (p<0.001) in Cx43-negative SANC. More samples are needed to confirm the results. Our current results indicated that, even though the AP firing rate or Ca2+ transient cycle length or LCR period are similar, the cells from central or peripheral SAN area are functionally different and the difference is not related to cell size. Is there any structural difference between the two cell types? Our preliminary data suggested that the immuno-labeling of Sodium-Calcium exchanger along the cell membrane in Cx43-negative SANC has a trend to be more intensive than in Cx43-positive SANC. How about other essential structural and functional proteins, like Serca2a, ryanodine receptors, and phospholamban? Is there any difference in terms of beta-AR stimulation, NCX inhibition or ryanodine receptor blockage? It's an ongoing project, and more experiments are designed to answer these fundamental questions.
