Example recordings of both caffeine-induced and Ca²⁺-activated force responses from a single cardiomyocyte bundle.

<p>(A) After skinning with 50ug.ml^-1 saponin for 25 mins, a bundle was exposed to a load /release protocol (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0138388#sec002" target="_blank">methods</a>) to ascertain the SR loading and Ca²⁺ storage properties. From left to right, the trace shows force responses to 30mM caffeine (solid line) after various load times (downward arrows). (B) The same bundle was skinned with Triton X-100 for 20 min, and the Ca²⁺ sensitivity and maximum Ca²⁺-activated force responses were then determined by exposing the bundles to heavily buffered Ca-EGTA solutions with increasing free [Ca²⁺] (pCa: 7.5, 5.7, 5.4, 5.2, 5.0, 4.5 as indicated by the upward arrows, respectively).