Gastric fundus smooth muscle (SM) contracts tonically (tension rises slowly and is maintained) rather than phasically (Kuriyama et al. 1975) so cytoplasmic calcium events may be different from those described so far in predominantly phasic SMs (e.g. Gordienko et al. 2001). Guinea-pigs were killed humanely by cervical dislocation, followed by exanguination. Enzymatically dispersed SM cells from the gastric fundus were studied using laser scanning confocal microscopy coupled with fluo-3 AM and tetramethyl-rhodamine methyl ester, which respond to ionised calcium, [Ca2+]i, and mitochondrial membrane potential, respectively.
SM cells could easily be recognised by their serpentine shape, smooth cell membrane and typical SM cell subcellular morphology (Gordienko et al. 2001) of dispersed strands of sarcoplasmic reticulum (SR) and mitochondria, with well-developed perinuclear SR at each nuclear pole. They rarely showed localised, spontaneous increases in cytosolic calcium (‘sparks’) but calcium waves occurred in response to caffeine (5 mM) or carbachol (10 µM).
Another type of cell was observed that shared the elongated shape of the SM cell, but was less serpentine in shape with numerous small ‘bumps’ on its membrane and which showed, often at three or more sites, periodically occurring localised, spontaneous increases in cytosolic calcium. These were about 10-15 µm wide and decayed exponentially with a time constant (τ) of about 1.5-2.0 s, much slower than is typically observed for well-characterised calcium increases in phasic guinea-pig intestinal SM cells (τ = 0.1-0.2 s). Often bunched trains of these events could be seen to produce sustained plateaux of elevated [Ca2+]i. These cells were rich in mitochondria, as are interstitial cells of Cajal (ICC; Ward et al. 2000) and displayed spontaneous changes in mitochondrial membrane potential, not seen in SM cells.
The absence of calcium ‘sparks’ in SM cells and the slower kinetics of the transient calcium events in ICC-like cells are being investigated in relation to the weakly phasic and strongly tonic contractile behaviour of the gastric fundus.This work was supported by The Wellcome Trust, grant no. 042293. S.P.P. is supported by the Medical Research Council.
- Gordienko, D.V., Greenwood, I.A. & Bolton, T.B. (2001). Cell Calcium 29, 13-28.
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Ward, S.M., Beckett, E.A.H., Wang, X.Y., Baker, F., Khoyi, M. & Sanders, K.M. (2000). J. Neurosci. 20, 1393-1403.