Figure 5: (a) Silver-stained gel showing the myosin heavy chain isoforms contained in a rat cardiac myocyte preparation compared to a pig cardiac myocyte preparation. (b) Representative length and force traces during a lightly loaded force clamp in a rat cardiac myocyte preparation (red) and a pig cardiac myocyte preparation (black) during a submaximal Ca2+ activation. (c) Length traces exhibited considerably greater curvature (greater ) in rat myocytes compared to pig myocyte preparations at all relative loads. Inset in C shows an expanded plot of versus relative load, which clarifies slope constants below 0.30. Interestingly, PKA-mediated phosphorylation increased the curvature () of length traces towards those of rat myocyte preparations. In addition, pig cardiac myocyte preparations that exhibited steep length-tension relationships (L-T R) also had more curved length traces. This is consistent with PKA-mediated phosphorylation of myofilaments yielding greater responsiveness to changes in sarcomere length, in this case exhibited by greater shortening-induced cooperative deactivation.