1. Thyroid hormone (L-thyroxine, 10−10mol/l) incubated in vitro with human erythrocyte membranes induced the release of a soluble calmodulin-like material, the 3′:5′-cyclic nucleotide phosphodiesterase-stimulating activity of which was at least six-fold greater than its concentration measured by a specific calmodulin radioimmunoassay.

2. The material had the characteristics of calmodulin in that it stimulated both phosphodiesterase and erythrocyte Ca2+-ATPase activities, cross-reacted with and was neutralized by anti-calmodulin antibody, was adsorbed by phenothiazine-Sepharose and was heat-stable. Control supernatant from the incubation of membranes in the absence of thyroxine contained calmodulin, the bioactivity of which was not enhanced beyond that predicted from radioimmunoassay. Subsequent addition of thyroxine did not increase calmodulin bioactivity. Calmodulin-agarose removed calmodulin-enhancing activity from the supernatant.

3. Thus, the enhancing factor(s) appears to interact directly with calmodulin. These observations indicate that thyroid hormone promotes the release from human erythrocyte membranes of a soluble factor (or factors) which binds to calmodulin and significantly increases its bioactivity. This enhancing activity is similar to that of a calmodulin activator described in a rat model of hypertension (S.-L. Huang et al., J Clin Invest 1988; 82: 276-81).

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