Characterization of Mg-ATP-dependent Ca2+ transport in cat pancreatic microsomes

Mechanisms of acinar cell injury in acute pancreatitis

Acute pancreatitis has many causes, all leading to a common pathway of changes within the pancreatic acinar cell. Key amongst these changes is premature intracellular activation of digestive enzymes but this is also accompanied by the appearance of cytosolic vacuoles, co-localization of digestive and lysosomal enzymes, activation of NF-kappaB, and release of pro-inflammatory cytokines. The exact mechanism responsible for enzyme activation remains the subject of much research effort and not a little debate, however it is clear that all of these changes are triggered by an abnormal, sustained rise in cytosolic calcium concentration, which is itself dependent both on release of calcium from endoplasmic reticulum stores and uptake from the extracellular milieu. Activated enzymes are directly damaging to the acinar cell themselves, but recruitment of circulating neutrophils leads to further cellular damage. Cytokines and neutrophil activation are also responsible for the systemic inflammatory response typically seen in severe acute pancreatitis.

The effects of Na+ replacement on intracellular pH and [Ca2+] in rabbit salivary gland acinar cells

1. The role of Na(+)-dependent mechanisms in regulating the intracellular pH (pHi) and free calcium concentration ([Ca2+]i) in acinar cells of the rabbit mandibular salivary gland was examined. The fluorescent dyes BCECF and Fura-2 were used to measure pHi and [Ca2+]i respectively in suspensions of isolated acini. 2. Replacement of all the extracellular Na+ with N-methyl-D-glucamine (NMDG) decreased resting pHi from a control value of 7.1-7.2 to 6.8-6.9. Re-addition of Na+ or Li+ caused a recovery of pHi towards control values. This recovery was blocked by 10-50 microM-ethylisopropylamiloride (EIPA), suggesting that it was mediated by Na(+)-H+ exchange. The rate of recovery of pHi when Na+ was re-introduced increased with Na+ concentration with an apparent Km for Na+ of around 30 mM. 3. Replacement of all of the extracellular Na+ with Li+ caused only a small decrease in resting pHi. 4. Stimulation of acini with 1 microM-acetylcholine (ACh) evoked an intracellular acidosis both under control conditions and when acini were bathed in Na(+)-free media. Following the acidosis pHi recovered in acini bathed in either control medium or Na(+)-free (Li+) medium, but not in acini bathed in Na(+)-free (NMDG) medium or in control medium containing EIPA. 5. Stimulation of acini bathed in Na(+)-free, HCO(3-)-free medium with ACh did not cause any change in pHi. 6. Re-addition of Na+ to acini bathed in Na(+)-free, HCO(3-)-free medium evoked the same rate of alkalinization whether or not the acini had been stimulated with ACh, suggesting that receptor stimulation per se did not lead to an activation of acid extrusion. 7. Resting [Ca2+]i was elevated in acini bathed in Na(+)-free (NMDG) medium, but not in acini bathed in Na(+)-free (Li+) medium. 8. ACh evoked a maintained rise in [Ca2+]i in acini bathed in control medium and in Na(+)-free media with either NMDG or Li+ as the Na+ substitute. 9. Experiments in which external Ca2+ was reduced to low levels (by the addition of EGTA) just prior to addition of ACh showed that ACh released intracellular Ca2+ stores under both control and Na(+)-free conditions. 10. In acini bathed in Na(+)-free (NMDG) solution and stimulated with ACh, re-addition of either Na+ or Li+ reduced [Ca2+]i. The reduction of [Ca2+]i on Na+ re-addition was blocked by EIPA. [Ca2+]i could also be reduced under these conditions by alkalinizing the cytosol using the weak base trimethylamine.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationship between Ca(2+)-transport and ATP hydrolytic activities in guinea-pig pancreatic acinar plasma membranes

Partially purified plasma membrane fractions were prepared from guinea-pig pancreatic acini. These membrane preparations were found to contain an ATP-dependent Ca(2+)-transporter as well as a heterogenous ATP-hydrolytic activity. The Ca(2+)-transporter showed high affinity for Ca2+ (KCa2+ = 0.04 +/- 0.01 microM), an apparent requirement for Mg2+ and high substrate specificity. The major component of ATPase activity could be stimulated by either Ca2+ or Mg2+ but showed a low affinity for these cations. At low concentrations, Mg2+ appeared to inhibit the Ca(2+)-dependent ATPase activity expressed by these membranes. However, in the presence of high Mg2+ concentration (0.5-1 mM), a high affinity Ca(2+)-dependent ATPase activity was observed (KCa2+ = 0.08 +/- 0.02 microM). The hydrolytic activity showed little specificity towards ATP. Neither the Ca(2+)-transport nor high affinity Ca(2+)-ATPase activity were stimulated by calmodulin. The results demonstrate, in addition to a low affinity Ca2+ (or Mg2+)-ATPase activity, the presence of both a high affinity Ca(2+)-pump and high affinity Ca(2+)-dependent ATPase. However, the high affinity Ca(2+)-ATPase activity does not appear to be the biochemical expression of the Ca(2+)-pump.

Calcium-ion-transporting activity in two microsomal subfractions from rat pancreatic acini. Modulation by carbamylcholine

Two microsomal subfractions from isolated rat pancreatic acini were produced by centrifugation through a discontinuous sucrose density gradient and characterized by biochemical markers. The denser fraction ( SF2 ) was a highly purified preparation of rough endoplasmic reticulum; the less-dense fraction ( SF1 ) was heterogeneous and contained Golgi, endoplasmic reticulum and plasma membranes. 45Ca2+ accumulation in the presence of ATP and its rapid release after treatment with the bivalent-cation ionophore A23187 were demonstrated in both fractions. The pH optimum for active 45Ca2+ uptake was approx. 6.8 for the rough endoplasmic reticulum ( SF2 ) and approx. 7.5 for SF1 . Initial rate measurements were used to determine the affinity of the rough-endoplasmic-reticulum uptake system for free Ca2+. An apparent Km of 0.16 +/- 0.06 microM and Vmax. of 21.5 +/- 5.6 nmol of Ca2+/min per mg of protein were obtained. 45Ca2+ uptake by SF1 was less sensitive to Ca2+, half-maximal uptake occurring at 1-2 microM-free Ca2+. When fractions were prepared from isolated acini stimulated with 3 microM-carbamylcholine, 45Ca2+ uptake was increased in the rough endoplasmic reticulum. The increased uptake was due to a higher Vmax. with no significant change in Km. No effect was observed on 45Ca2+ uptake by SF1 . In conclusion, two distinct non-mitochondrial, ATP-dependent calcium-uptake systems have been demonstrated in rat pancreatic acini. One of these is located in the rough endoplasmic reticulum, but the precise location of the other has not been determined. We have shown that the Ca2+-transporting activity in the rough endoplasmic reticulum may have an important role in maintaining the cytosolic free Ca2+ concentration in resting acinar cells and is involved in Ca2+ movements which occur during stimulation of enzyme secretion.