1 68 



HANDBOOK OF PHYSIOLOGY ~^ CIRCULATION I 



TABLE I . Calcium Partition in Muscle 



TABLE 2. Ca^^ Equilibration in Tissues 



Rapid 

 Ca" 

 efflux 



Slow Ca" 

 efflux 



No Ca" 

 efflux 



/iM Ca/gm 

 Connective Tissue Space 



1. Aqueous* .16 



2. Bound to connective tissuef .25 



Muscle Fibers 



3. Surface bound| o.i + 0.03 



4. Myoplasm, exchangeable .33 



5. Myoplasm, nonexchange- Not deter- 



able mined 



* This represents the Ca''^ in the extracellular fluid. The 

 extracellular fluid volume was based on a sucrose space of 

 16%. t This fraction was based on Ca'* distribution in 



tendon and the assumption that the tendon and muscle con- 

 nective tissue were comparable with respect to calcium 

 binding capacity. J This represents the total surface- 

 bound Ca'* less the amount bound to connective tissue. The 

 0.03 figure is calculated from the sudden increase in Ca'" 

 efflux which occurs when the tissue is shifted from a calcium- 

 free medium into a solution containing calcium. It is thought 

 to be a superficial moiety of "self-exchanging" calcium 

 which is capable of exchanging with any calcium that exists 

 in the bathing fluid but which is rather firmly bound to sur- 

 face sites in the absence of calcium. 



analysis of an experimentally obtained calcium 

 uptake curve of the muscle. A .somewhat more com- 

 plete analysis along the same lines was made by 

 Shanes & Bianchi (272). In their experiments Ca^* 

 efflux curves from frog sartorius muscles were ob- 

 tained. Their results are presented in table i. The 

 efflux curve is divided into a fast and slow phase. 

 The fast phase is considered to represent / ) Ca''° in 

 the extracellular fluid; _') a moiety bound to connec- 

 tive tissue; and j) a so-called surface-bound calcium 

 which, since the connective tissue space moiety is 

 accounted for in 2, is assumed to be bound to the 

 surface of the muscle fibers. The rest of the tissue 

 calcium is, then, in the myoplasm, and this accoimts 

 for the slowly exchanging muscle calcium. A fraction 

 of the tissue calcium is nonexchangeable under the 

 conditions of the experiment. 



Harris (120), on the basis of iontophoresis experi- 

 ments, also concluded that calcium ions become 

 bound to surface sites of skeletal muscle because of 

 its low mobility in an electric field. 



The studies noted above indicate that calcium is 

 bound to connective tissue and quite likely muscle 

 surface. There is also a good deal of cjualitative 

 evidence to suggest that calcium is bound to intra- 

 cellular protoplasmic components as well. It has 

 already been noted that cut muscle binds more 



Tissue 

 Rabbit atrium 

 Squid axoplasm 

 Frog skeletal muscle 

 Frog skeletal muscle 

 Frog skeletal muscle 



Rat skeletal muscle 



Equilibration. 

 % of Extracellular 

 Reference Specific Activity 



(267 ) 2-4% in 4 hr 



(146) 20% in 81 2 hr 



(272) 38% in 4 hr 



(120) 10-25% in 16-20 hr 



(57) 45% in 6 hr 



25% in Yi-^ hr* 



100% in 6-10 hrf 



(57) 100% in 2 hrj 



* In vivo, resting frogs. f In vivo, jumping frogs. 



X In vivo, active rats. 



calcium than intact muscle (92). This has also been 

 found for breis of frog muscle (317); and it has been 

 reported that no ultrafiltrable calcium can be ob- 

 tained from a suspension of finely cut up rat muscle 

 (318). An attempt to construct a more quantitative 

 picture of the state of calcium within living tissue 

 has been made by Hodgkin & Keynes (146) who 

 studied the movement of a microvolume of Ca''* 

 injected into the axoplasm of an intact squid nerve. 

 They found that the mobility of the injected calcium 

 was less than i 45 of what it would have been in free 

 solution. Their conclusion is that the ratio of ionized 

 to total calcium in the cell cannot be more than 

 about 0.02 and assuming a total axon calcium concen- 

 tration of 0.4 mM per kg the free calcium concentra- 

 tion in squid nerve should be less than 0.0 1 mM per 

 kg. * Hodgkin and Keynes argue that if calcium were 

 distributed according to the Nernst equation, the 

 ratio of intracellular to extracellular calcium should 

 be more than 100. The actual value for squid axon 

 derived from their experiments would be about 

 0.01 1 1, so that clearly the system is far from equili- 

 brium with respect to calcium. A similar viewpoint is 

 presented by Gilbert & Fenn (92). It would thus 

 appear both from this reasoning and from the 

 observations that damaged tissues accumulate cal- 

 cium, that the cell interior maintains a very low free 

 calcium ion concentration compared to its environ- 

 ment. Not only does the cell membrane have a low 

 permeability to calcium ion (as will be elucidated in 

 the next section) but the cell must have the capacity 



' One might contend that the micro-injection of calcium into 

 the cytoplasm of a living cell would not provide an accurate 

 reflection of the mobility of the naturally occurring calcium. 

 It is known for example that such injections cause gelation or 

 clotting of the cytoplasm of certain tissues; however, in the case 

 of the squid axon the calcium apparently liquefies the squid 

 axoplasm. For references on this interesting subject the reader 

 is referred to the paper of Chambers & Kao (49). 



