Abstract
The steady state relationship between intra- and extramitochondrial free Ca2+ across the inner mitochondrial membrane has been investigated in isolated liver mitochondria. The extramitochondrial free Ca2+ concentration was essentially independent of the mitochondrial calcium content above 4 nmol/mg of protein. Below this value, a decrease in the mitochondrial calcium content was accompanied by a decrease in the extramitochondrial free Ca2+ concentration. The experimental data are compatible with a model in which the steady state distribution of calcium is described in terms of the kinetic parameters of the separate carriers catalyzing Ca2+ influx and efflux across the mitochondrial inner membrane. The corresponding relationship between cytosolic free Ca2+ concentration and the amounts of calcium in the mitochondria and endoplasmic reticulum was investigated in isolated river cells over a range of cellular Ca2+ contents by using a nondisruptive technique based on the selective release of calcium from mitochondrial and total cellular pools by addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone and A23187, respectively. A net increase in cell calcium from 1 to 5 nmol/mg dry weight, increased the cytosolic free Ca2+ concentration from 0.1 to about 0.3 microM and increased the calcium contents of both mitochondria and endoplasmic reticulum. Above 5 nmol of calcium/mg cell dry weight, the endoplasmic reticulum calcium pool became filled, and further increases in calcium content were accounted for by increases of the mitochondrial pool but no further increase of the cytosolic free Ca2+ concentration. These studies and experiments with mixtures of isolated microsomes and mitochondria suggest that, in cells as normally isolated (containing 5 to 6 nmol of calcium/mg dry weight), the endoplasmic reticulum is saturated with calcium and is unlikely to play a major role as an intracellular calcium buffer. The in situ mitochondrial calcium content is sufficiently high (approximately 16 nmol/mg of protein) for these organelles to buffer effectively the cytosolic free Ca2+ concentration at a value of about 0.3 microM. In addition, it may be concluded that intramitochondrial Ca2+-dependent enzymes will be exposed to saturating concentrations of free Ca2+.
Highlights
The steady state relationshipbetween intra- andex- appear to cause mobilization of intracellular calcium (6-9), tramitochondrial free Ca2+ across the innemritochon- which, through allosteric activation of phosphorylase kinase drial membrane has been investigated in isolated liver(10,ll),leads to increased glycogenolysis
As found by other workers (24, 27), the extramitochondrial free Ca2’ concentration maintained by the mitochondria in the steady state was relatively constant following addition of four aliquots of 2 nmol of Ca2+/mg of protein
When the same calcium additions were made to mitochondria which had been depleted of calcium to a level of 1.5 nmol/mg of protein, there was a progressive increase in the steady state extramitochondrial free Ca2+concentration (Fig. l B ),indicating that mitochondria buffered the extramitochondrial free Ca2+concentration poorly when they were calcium-depleted
Summary
The steady state relationshipbetween intra- andex- appear to cause mobilization of intracellular calcium (6-9), tramitochondrial free Ca2+ across the innemritochon- which, through allosteric activation of phosphorylase kinase drial membrane has been investigated in isolated liver(10,ll),leads to increased glycogenolysis. By use of calcium electrodes to monitor the extramitochondrial free Ca2‘ concentration, it has been shown that liver mitochondria as normally prepared will either take up orrelease calciuminresponse to small additions of Ca2+ or EGTA‘ without a significant change of the steady state extramitofectively the cytosolic freeCa2+concentration at a chondrial free Ca” concentration (24,27). These studiewsere value of about 0.3 pM.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
