Ds. Steele et al., EFFECTS OF CREATINE-PHOSPHATE AND INORGANIC-PHOSPHATE ON THE SARCOPLASMIC-RETICULUM OF SAPONIN-TREATED RAT-HEART, Journal of physiology, 483(1), 1995, pp. 155-166
1. Ventricular trabeculae from rat heart were permeabilized by treatme
nt with saponin. In the presence of 150 nM Ca2+, application of 20 mM
caffeine released Ca2+ from the sarcoplasmic reticulum (SR), resulting
in a transient contracture. Ca2+ released from the SR was detected us
ing fura-2 fluorescence. The amplitudes of the caffeine-induced Ca2+ t
ransients were used to assess SR Ca2+ content. 2. In the absence of cr
eatine phosphate (CP), introduction of 5-30 mM inorganic phosphate (P-
i) caused a net release of Ca2+ from the SR. Subsequent caffeine-induc
ed Ca2+ and tension transients were smaller in the presence of P-i. Un
der these conditions, 30 mM P-i decreased the caffeine-induced Ca2+ tr
ansients by 45 +/- 3.1% (mean +/- S.D., n = 14). On removal of P,(i) t
he [Ca2+] transiently decreased and the caffeine-induced Ca2+ transien
ts returned to control levels over 4-6 min. 3. In the presence of CP (
5-15 mM), the Ca2+ transients were unaffected by the introduction of P
-i (5-30 mM) or slightly increased in amplitude. P-i (30 mM) significa
ntly increased the caffeine-induced Ca2+ transients by 7 +/- 8.8% (mea
n +/- S.D., n = 19, P < 0.05) in the presence of 15 mM CP. The release
of Ca2+ on addition of P-i and decrease in [Ca2+] on P-i withdrawal w
as less pronounced or absent completely in the presence of CP. The inh
ibitory effects of P-i on caffeine-induced Ca2+ release became apparen
t as the [CP] was decreased from 5 to 0 mM 4. In the presence of the c
reatine phosphokinase inhibitor dinitro-fluorobenzene (DNFB) the effec
ts of P-i (in the presence of CP) were qualitatively similar to the re
sults obtained in the absence of CP, although the decrease in caffeine
-induced Ca2+ release was less pronounced. 5. These results suggest th
at the rise in [P-i](i) during ischaemia or anoxia will have little ef
fect on the regulation of Ca2+ by the SR while the [CP](i) remains abo
ve 5 mM. However, as the [CP] decreases below 5 mM, the accumulation o
f P-i within the cytosol will progressively reduce the SR Ca2+ content
. CP may act in conjunction with endogenous creatine phosphokinase to
modify the response of the SR to P-i, and possible mechanisms are cons
idered.