The Superficial Buffer Barrier Hypothesis

In addition to this function of controlling the overall cytosolic Ca concentration adjacent to the myofilaments in the deep cytosol, there is evidence that the st also plays a role in controlling the influx of extracellular Ca - through the plasmalemmal membrane. This has led to the independent proposal of two concepts - the superficial buffer barrier hypothesis and the capacita-tive model - both of which attempt to link sr function to extracellular Ca influx. 

An additional model which aims to integrate observations that the Ca content of the sr regulates Ca fluxes across the plasmalemma is the capacitative model , expounded by Putney (1986, 1990). 

It would be naive to think that at any one time ASM is either in a contracted or a relaxed state. ASM in vivo has a degree of tone, and the extent of this tone will depend on the balance between the mechanisms producing contraction and relaxation. Relaxation, therefore, is relative, and results both from a reduction in the forces of con- 

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