Calcium ion in muscle

Calcium plays an important part in structure-building in living organisms, perhaps mainly because of its ability to link together phosphate-containing materials. Calcium ions in the cell play a vital part in muscle contraction. 

FIGURE 2.19 Potentiation and modulation of response through control of cellular processes, (a) Potentiation of inotropic response to isoproterenol in guinea pig papillary muscle by the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX). Ordinates percent of maximal response to isoproterenol. Abscissa percent receptor occupancy by isoproterenol (log scale). Responses shown in absence (open circles) and presence (filled circles) of IBMX. Data redrawn from [7], (b) Effect of reduction in calcium ion concentration on carbachol contraction of guinea pig ileum. Responses in the presence of 2.5 mM (filled circles) and l.5mM (open circles) calcium ion in physiological media bathing the tissue. Data redrawn from [8],... 

Carnosine is an avid chelator of metal ions (Baran, 2000). Complexes with calcium, copper, and zinc ions have been described (Trombley et ah, 2000). It is possible, therefore, that carnosine could exert some sort of control of calcium metabolism in muscle tissue (heart or skeletal). It is also likely that the dipeptide controls the availability of zinc ions in neuronal tissue, especially the olfactory lobe where both carnosine and zinc are enriched (Bakardjiev, 1997 Bonfanti et ah, 1999 Sassoe-Pognetto et ah, 1993). Zinc-camosine complexes, called polaprezinc, are also effective in the repair of ulcers and other lesions in the alimentary tract (Matsukura and Tanaka, 2000). 

Another group of drugs that relax heart muscle are the calcium-channel blockers. One example is nifedipine, shown in Figure 14.46. Muscle contraction is initiated as a nerve impulse signals calcium ions to enter muscle cells. As their name implies, calcium-channel blockers inhibit the flow of calcium ions into muscles, thereby inhibiting muscle contraction. The heart rate slows down, and muscles of blood vessels relax and dilate, lowering blood pressure. 

Contraction-relaxation processes in muscle proceed by a sliding filament mechanism, whereby actin and myosin filament move relative to one another. The reaction is energized by ATP and regulated by the level of calcium ions. In vertebrate skeletal muscle contraction is controlled by the interaction of calcium ions with the specific protein troponin which is attached to the actin filaments, whereas among many invertebrates troponin is lacking and myosin and not actin is controlled192. However, in a few invertebrates both types of filaments are involved in regulation. 

Calcium ions are important in muscle contraction and in regulating heartbeat. If the concentration of calcium ions falls too low, death is inevitable. In a television drama, a patient is brought to hospital after being accidentally splashed with hydrofluoric acid. The acid readily penetrates the skin, and the fluoride ions combine with the calcium ions in the blood. If the patient s volume of blood plasma is 2.8 L, what amount (in mol) of fluoride ions would completely combine with all the calcium ions in the patient s blood ... 

LaBelle EF, Fulbright RM, Barsotti RJ, Gu H, Polyak E. Phospholipase D is activated by G protein and not by calcium ions in vascular smooth muscle. Am J Physiol Heart Circ Physiol 1996 39 H1031-H1037. 

We will consider the structural and mechanistic features of P-type ATPases by examining the Ca ATPase found in the sarcoplasmic reticulum (SR Ca ATPase, or SERCA) of muscle cells. The properties of this member have been established in great detail, by relying on crystal structures of the pump in five different states. This enzyme, which constitutes 80% of the protein in the sarcoplasmic reticulum membrane, plays an important role in muscle contraction, a process triggered by an abrupt rise in the cytoplasmic calcium ion level. Muscle relaxation depends on the rapid removal of from the cytoplasm into the sarcoplasmic reticulum, a specialized compartment for Ca storage, by SERCA, This pump maintains a Ca" concentration of approximately 0.1 pM in the cytoplasm compared with 1.5 mM in the sarcoplasmic reticulum. 

B. Hyperparathyroidism is the likely cause of all of the patient s symptoms. Increased parathyroid hormone leads to bone demineralization, increased calcium uptake from the intestine, increased blood levels of calcium, decreased calcium ion excretion by the kidney, and increased phosphate excretion in the urine. Increased blood calcium levels caused renal stones, while bone demineralization progressed to osteopenia. The patient s intake of calcium and vitamin D are not excessive. Calcitonin acts to decrease bone demineralization. Muscle weakness and depression reflect the widespread role of calcium ion in many physiologic processes. 

Compared with normal muscle, dystrophic muscle exhibits delayed relaxation suggesting a disturbance of the excitation-contraction coupling mechanism. This process involves the release of calcium from the sarcoplasmic reticulum when the membrane becomes depolarised in response to a nerve impulse and the calcium ions in contact with the myofibrils cause them to contract. The muscle reverts to a relaxed condition when the calcium ions are removed by sequestration within the sarcoplasmic reticulum, a process that depends upon the active transport of calcium mediated by Ca -activated ATPase in conjunction with a repolarisation of the membrane. 

Although only small amounts of these drugs are absorbed, osmotic laxatives should not be used in patients with poor renal function because of the risk of magnesium accumulation. High concentrations of magnesium in the body can interfere with the function of calcium ions in the heart, skeletal muscle and the central nervous system. Effects of this include neuromuscular block or central nervous system depression. 

Weiss GB and Goodman ER (1975) Interactions between several rare earth ions and calcium ion in vascular smooth muscle. J Pharmacol Exp Ther 195 557-564. 

---