Regulation of cardiac Kv1.5 K+ channel expression by cardiac fibroblasts and mechanical load in cultured newborn rat ventricular myocytes.

Of the six voltage-gated K+ channel alpha subunits detected in rat heart, the Kv1.5 channel is abundantly expressed, and its gene transcription and protein expression are reduced during cardiac remodeling. Since cardiac fibroblasts and mechanical load have been known to play important roles in myocardial hypertrophy, we studied the regulation of Kv1.5 K+ channel protein expression by these factors in cultured newborn rat ventricular myocytes, using immunofluorescent cytochemistry and Western blot analysis. Ventricular cells were isolated from 1-day-old Wistar rats and cultured for a period of 5 days. The effect of cardiac fibroblasts was examined by co-culturing myocytes with fibroblasts or incubating pure myocytes in fibroblast-conditioned growth medium (FCGM) for 72 h. In addition, a 48-h cyclic stretch at 0.5 Hz with 20% elongation in length was applied to pure myocyte cultures to mimic mechanical load. With a polyclonal antibody against rat Kv1.5 K+ channel protein, single cultured myocytes showed a weak and uniform antibody labeling. Co-culturing with fibroblasts or incubating pure myocytes in FCGM both induced a significant increase in myocyte size implying cell hypertrophy, but neither allowed normal expression of the Kv1.5 K+ channel as indicated by almost negative anti-Kv1.5 labeling. Western blots of cell proteins prepared from ventricular myocyte cultures revealed a single protein band at 75 kD recognized by the anti-Kv1.5 antibody and a 45% decrease in Kv1.5 immunoreactive protein level in the FCGM-treated preparations. Application of 1 microM losartan, an angiotensin II type I receptor blocker, significantly attenuated the FCGM-induced myocyte hypertrophy and reduction of Kv1.5 K+ channel expression. On the other hand, although no cell hypertrophy was stimulated by mechanical stretch, intense punctate antibody labeling with a 48% increase in Kv1.5 protein level was observed in the stretched myocytes. These results suggest that the protein expression of cardiac Kv1.5 K+ channel is differentially regulated by cardiac fibroblasts and mechanical load. Some soluble factors produced from cardiac fibroblasts contribute to the depressed Kv1.5 K+ channel expression in myocardial hypertrophy. This channel regulation may be mediated by angiotensin II type I receptor.