Peripheral couplings are junctions between the sarcoplasmic reticulum (SR) and the surface membrane (SM). Feet occupy the SR/SM junctional gap and are identified as the SR calcium release channels, or ryanodine receptors (RyRs). In cardiac muscle, the activation of RyRs during excitation-contraction (e-c) coupling is initiated by surface membrane depolarization, followed by the opening of surface membrane calcium channels, the dihydropyridine receptors (DHPRs). We have studied the disposition of DHPRs and RyRs, and the structure of peripheral couplings in chick myocardium, a muscle that has no transverse tubules. Immunolabeling shows colocalization of RyRs and DHPRs in clusters at the fiber's periphery. The positions of DHPR and RyR clusters change coincidentally during development. Freeze-fracture of the surface membrane reveals the presence of domains (junctional domains) occupied by clusters of large particles. Junctional domains in the surface membrane and arrays of feet in the junctional gap have similar sizes and corresponding positions during development, suggesting that both are components of peripheral couplings. As opposed to skeletal muscle, membrane particles in junctional domains of cardiac muscle do not form tetrads. Thus, despite their proximity to the feet, they do not appear to be specifically associated with them. Two observations establish the identify of the structurally identified feet arrays/junctional domain complexes with the immunocytochemically defined RyRs/DHPRs coclusters: the concomitant changes during development and the identification of feet as the cytoplasmic domains of RyRs. We suggest that the large particles in junctional domains of the surface membrane represent DHPRs. These observations have two important functional consequences. First, the apposition of DHPRs and RyRs indicates that most of the inward calcium current flows into the restricted space where feet are located. Secondly, contrary to skeletal muscle, presumptive DHPRs do not show a specific association with the feet, which is consistent with a less direct role of charge movement in cardiac than in skeletal e-c coupling.

Membrane excitation was the basis for backward swimming of Paramecium facing stimulus. According to standard genetic tests, inexcitable mutants fell into three complementation groups for both Paramecium tetraurelia (pwA, pwB, and pwC) and Paramecium caudatum (cnrA, cnrB, and cnrC). Cytoplasm from a wild type transferred to a mutant through microinjection restored the excitability. Transfusions between genetically defined complementation groups of the same species effected curing, whereas transfusions between different mutants (alleles) of the same group or between sister cells of the same mutant clone did not. Cytoplasmic transfers of all combinations among the six groups of mutants of the two species showed that any cytoplasm, except those from the same group, was able to cure. Since the pawns and the caudatum nonreversals complement one another through transfusion, they appeared to belong to six different complementation groups. The extent of curing, the amount of transfer needed to cure, and the time course of curing were characteristic of the group that received the transfusion. Variations in these parameters further suggested that the six groups represented six different genes. Because the donor cytoplasms from either species were equally effective quantitatively in curing a given mutant, the curing factors were not species specific. These factors are discussed.

When cells of the behavioral mutant cnrC of Paramecium caudatum were mated with the wild type, phenotype change from CNR (no backward swinning) to wild type in the cnrC mate occurred immediately after the formation of tight pairs. No change of phenotype occurred when cells of cnrA or cnrB were mated with wild type. Phenotypic change from CNR to wild type in cells of cnrC was also induced by microinjection of wild-type cytoplasm. Microinjection of wild-type cytoplasm induced no change in cells of cnrA or cnrB. Phenotypic change in the cnrC mate during conjugation can be explained by cytoplasmic exchange during conjugation, though transfer of membrane sites for excitability through membrane fluidity cannot be ruled out.