OBJECTIVESActive mRNA distribution in the form of ribonucleoprotein particles moving along microtubules has been shown in several cell types, but not yet in cardiocytes. This study addresses two hypotheses: 1) a similar mRNA distribution mechanism operates in cardiocytes; 2) decoration of microtubules with microtubule-associated proteins compromises this distribution.METHODSTo visualize ribonucleoproteins in cultured neonatal rat cardiocytes, they were transfected with vectors encoding zipcode binding protein-1 and Staufen fused with GFP. The velocity of microtubular transport and elongation were calculated on time-lapse confocal pictures.RESULTSZBP-1 and Staufen labeled particles co-localized with each other and with microtubules and moved along microtubules over a distance of 1-20 microm with a mean speed of 80 nm/s. The average speed decreased about 50% after decoration of microtubules by adenoviral microtubule-associated protein-4 (MAP-4). The elongation speed measured using the GFP-tagged end-binding protein-1 exceeded 200 nm/s and was not influenced by MAP-4.CONCLUSIONSWe demonstrate for the first time ribonucleoprotein particles in cardiocytes, their microtubular-related movement, and its inhibition (but not of the microtubular elongation), by the MAP-4 decoration of microtubules.