In search for evidence in support of our hypothesis that the PF is enriched with miRNAs released from the heart and thoracic vasculature, under ethical approval, we obtained the PF, peripheral blood-derived plasma, and leftover tissue samples of myocardium (right atrial appendage) and vasculature (ascending thoracic aorta) from surgical patients undergoing aortic valve replacement (AVR) (for study population, see Table S1). An exploratory PCR-based miRNA microarray (Exiqon) on non-pooled, randomly selected samples of whole PF (n = 3) revealed the presence of several miRNAs in the PF of the patients (see Table S2). Interestingly, several miRNAs of putative cardiovascular origin appear to be relatively highly expressed (see red text in Table S2). Fifteen of these miRNAs were rationally selected for further analyses, together with miR-208 (supposedly enriched in cardiomyocytes) and the liver-enriched miR-122 (used as a “non-cardiovascular” control) (see Table S3 for the 16 cardiovascular miRNAs that were selected for further investigation and reporting the miRNA particulars). Working on surgical leftover samples, we validated that the 16 bona fide cardiovascular miRNAs were indeed well expressed in the myocardium and/or vasculature of the patients. By contrast and as expected, miR-122 was poorly expressed in the tissue samples (Figure S2). Next, by performing PCR for individual miRNAs on matched PF and plasma samples, we obtained evidence supporting the concept that the PF is enriched with miRNAs released from cardiovascular tissues (Figure 1; Supporting Materials and Methods). These data align with our hypothesis that the PF represents a liquid compartment in which expressional information and executive command from the heart and thoracic vessels are released in the form of miRNAs.Figure 1Human PF Is Enriched with MicroRNAs of Potential Cardiovascular OriginThe expression of selected microRNAs (miRNAs) was measured (RT-qPCR) in paired PF (black column) and peripheral plasma (gray column) samples. The PF/plasma concentration ratios of individual miRNAs are plotted on the right side of the graphs. Exogenous spike-in cel-miR-39 was the normalizer. MiR-122-5p was used as a non-cardiovascular control. N.D., not detectable. All values are mean + SEM. *p ≤ 0.05, **p ≤ 0.01, and ***p ≤ 0.001 versus plasma (unpaired Student’s t test); n = 5–9.