The geometry of the EJSR Z-rete in avian cardiac muscle
The junctional SRs (JSR) in skeletal and cardiac muscle in general, including extended junctional SR (EJSR) in avian hearts, are morphologic and functional homologies as shown by biochemical and morphometric evidence, including the recent demonstration that EJSR contains ryanodine receptors and binds [3H]-Ryanodine. Bird hearts have two aberrant membrane features: 1. absence of transverse tu-bules (TT) and, 2. extension of JSRs (thus: E-JSR) from peripheral couplings into the interior of the myocytes forming EJSR Z-retes that both surround and pervade Z-discs. Absence of TT calls for an alternative mechanism for global muscle activation in bird hearts for which EJSR may provide the anatomical substratum, a possibility supported by both experimental evidence, and recent theoretical considerations. Propagated calcium-induced calcium release (CICR) has been difficult to reconcile with the graded response of cardiac muscle to stimulation.2,cf.5 Propagation of an action potential along the Z-retes can be excluded. In the absence of TT, saltatory CICR as an alternative to propagated cardiac activation, is made plausible by the existence of EJSR which, (a) is demonstrably calcium-sensitive, (b) is well within < than 0.5μm of the regulatory proteins of cardiac myocytes and, (c) carries rows of proven, vicinal calcium release channels.