| dc.description | Positive inotropy following rest, in elevated extracellular calcium (Ca 2+), during hypothermia or with increasing stimulation frequency has been attributed to increased sarcoplasmic reticulum (SR) Ca2+ content and/or increased trigger Ca2+. However, each of these manipulations also may elevate diastolic [Ca2+]. The objective of this study was to determine whether diastolic [Ca2+] could contribute to positive inotropy in isolated guinea pig ventricular myocytes at physiological temperatures (37°C). Intracellular [Ca2+] was measured with fura-2. Cell shortening was measured with an edge detector. SR Ca2+ stores were assessed by rapid application of 10 mM caffeine. Under voltage clamp conditions, when extracellular [Ca2+ ] was elevated contraction amplitude increased with increasing extracellular [Ca2+], in parallel with increasing diastolic and peak systolic Ca2+. Interestingly, Ca2+ transient amplitude did not increase. Furthermore, SR Ca2+ stores did not increase when extracellular [Ca2+] was elevated. Thus, positive inotropy occurred in the absence of changes in Ca2+ transient amplitude, indicating that diastolic Ca2+ may be an important contributor to positive inotropy. When voltage clamped myocytes were stimulated from rest similar results were found; diastolic Ca2+ and contraction amplitude increased while Ca2+ transient amplitude did not. Thus, these data suggest that diastolic Ca2+ is an important determinant of contraction amplitude. In contrast, both Ca2+ transient amplitude and diastolic Ca2+ increased in parallel with increasing contraction amplitude when rates of field stimulation were increased. Thus, when Ca2+ transient amplitude and diastolic Ca2+ both increased, positive inotropy occurred. In hypothermic myocytes (24°C) amplitudes of contractions, SR Ca 2+ stores and Ca2+ transients were increased with respect to 37°C, while diastolic Ca2+ did not change. Therefore positive inotropy also occurred when Ca2+ transient amplitude increased, in the absence of increasing diastolic Ca2+. Thus, the data presented in this thesis demonstrate that a change in either diastolic Ca2+ or Ca2+ transient amplitude was sufficient for positive inotropy to occur. Interestingly, when either diastolic Ca 2+ or Ca2+ transient amplitude increased, the peak systolic [Ca2+] achieved also was increased. Therefore, by influencing peak systolic Ca2+ both Ca2+ transient amplitude and diastolic Ca2+ could influence contraction amplitude. | en_US |
