Phosphagens phosphocreatine

The most dramatic illustration of a mass-specific illusion is the comparative heat dissipation of the human erythrocyte and platelet. In mammals, both of these cell types are anucleate and discoid in shape, but the longest dimension of the former is four times that of the latter. Yet heat production of a human erythrocyte was shown to be 10 fW, a sixth that of a human platelet (61 fW see Table 1). The relatively high metabolic activity of platelets is probably due to the need to maintain a considerable phosphagen (phosphocreatine) pool for actomyosin contraction at stimulation and clot retraction. Phosphocreatine is synthesized from creatine using ATP and acts as a demand on the ATP cycle to drive the coupled catabolic half-cycle. On the other hand, ATP requirements of the erythrocyte are relatively small, being mostly confined to active transport of ions at the plasma membrane. 

During periods of rest when ATP is abundant, creatine is phos-phorylated by creatine kinase to form phosphocreatine. This reaction is especially important in muscles. When a sudden explosive burst of muscle activity occurs, phosphocreatine phosphorylates adenosine diphosphate (ADP) to generate the ATP needed for muscle contraction (Fig. 10.4). For this reason, phosphocreatine is known as a phosphagen . 

Labile Phosphate of Muscular Contraction.— Phosphagens, including phosphocreatine, phosphoarginine and adenosine triphosphate. 

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