Systematic immunological and biochemical studies indicate that the level of expression of sarcoplasmic reticulum (SR) Ca(2+)-ATPase regulatory protein phospholamban (PLB) in mammalian slow-twitch fibers varies from zero, in the rat, to significant levels in the rabbit, and even higher in humans. The lack of PLB expression in the rat, at the mRNA level, is shown to be exclusive to slow-twitch skeletal muscle, and not to be shared by the heart, thus suggesting a tissue-specific, in addition to a species-specific regulation of PLB. A comparison of sucrose density-purified SR of rat and rabbit slow-twitch muscle, with regard to protein compositional and phosphorylation properties, demonstrates that the biodiversity is two-fold, i.e. (a) in PLB membrane density; and (b) in the ability of membrane-bound Ca(2+)-calmodulin (CaM)-dependent protein kinase II to phosphorylate both PLB and SERCA2a (slow-twitch isoform of Ca(2+)-ATPase). The basal phosphorylation state of PLB at Thr-17 in isolated SR vesicles from rabbit slow-twitch muscle, colocalization of CaM K II with PLB and SERCA2a at the same membrane domain, and the divergent subcellular distribution of PKA, taken together, seem to argue for a differential heterogeneity in the regulation of Ca(2+) transport between such muscle and heart muscle.