Figure 4.

(A) Effect of cAMP on the relationship between isometric force and [Ca²⁺] in permeabilized ABRM. •, control; ○, + cAMP. See text for details of protocol. Force responses were normalized to the maximum force at pCa 5 + cAMP. (B) Effect of cAMP on Vo at pCa 6 in permeabilized ABRM. Vo was determined during steady-state force maintenance after activation in pCa 6. The muscle then was treated with cAMP, which caused partial relaxation. Calcium was added to the bathing medium until the force matched the initial force in pCa 6 alone, and Vo was determined again. The force responses and Vo were normalized to the initial responses in pCa 6 for each muscle (n = 15).