Wilkens, J.L.; Yazawa, T.; Shinozaki, T.; ter Keurs, HEDJ: Sites and mode of action of proctolin and F2 on lobster cardiac muscle

At low concentration (10^-10 M), the peptide hormones proctolin (PR) and F2 cause an increase in the amplitude of electrically evoked contractions of lobster heart ostial valve muscle. At higher concentrations (PR >10^-9 M, F2 >10^-7 M), each peptide also induces contracture. The increases in contracture and contraction force are proportional to increases in [Ca²⁺]_i. The prolonged elevation of [Ca²⁺]_i during PR-induced contracture causes a right shift in the force-pCa curve indicating a decrease in myofibrillar sensitivity to Ca²⁺. Reducing [Ca²⁺]_o or blocking Ca²⁺ channels with Cd²⁺ or nifedipine reduced contractile force and PR responses. Blocking sarcoplasmic reticular Ca²⁺ release with ryanodine reduced phasic contractions but did not prevent PR-induced contracture. A number of inhibitors of protein kinase A and C did not alter PR or F2 effects. We conclude that PR and F2 bind to sarcolemmal receptors to increase Ca²⁺ currents. Neither peptide appears to act via intracellular signal transduction pathways.