Origin of slow and fast oscillations of Ca²⁺ in mouse pancreatic islets

Abstract

Pancreatic islets exposed to 11 mM glucose exhibited complex variations of cytoplasmic Ca2+ concentration ([Ca2+]i) with slow (0.3‐0.9 min−1) or fast (2‐7 min−1) oscillations or with a mixed pattern. Using digital imaging and confocal microscopy we demonstrated that the mixed pattern with slow and superimposed fast oscillations was due to separate cell populations with the respective responses. In islets with mixed [Ca2+]i oscillations, exposure to the sarcoplasmic‐endoplasmic reticulum Ca2+‐ATPase inhibitors thapsigargin or 2,5‐di‐tert‐butylhydroquinone (DTBHQ) resulted in a selective disappearance of the fast pattern and amplification of the slow pattern. In addition, the protein kinase A inhibitor RP‐cyclic adenosine 3′,5′‐monophosphorothioate sodium salt transformed the mixed [Ca2+]i oscillations into slow oscillations with larger amplitude. Islets exhibiting only slow oscillations reacted to low concentrations of glucagon with induction of the fast or the mixed pattern. In this case the fast oscillations were also counteracted by DTBHQ. The spontaneously occurring fast oscillations seemed to require the presence of cAMP‐elevating glucagon, since they were more common in large islets and suppressed during culture. Image analysis revealed [Ca2+]i spikes occurring irregularly in time and space within an islet. These spikes were preferentially observed together with fast [Ca2+]i oscillations, and they became more common after exposure to glucagon. Both the slow and fast oscillations of [Ca2+]i in pancreatic islets rely on periodic entry of Ca2+. However, the fast oscillations also depend in some way on paracrine factors promoting mobilization of Ca2+ from intracellular stores. It is proposed that such a mobilization in different cells within a tightly coupled islet syncytium generates spikes which co‐ordinate the regular bursts of action potentials underlying the fast oscillations.

Disorganization of cytoplasmic Ca 2+ oscillations and pulsatile insulin secretion in islets from ob / ob mice

“…The experiments were carried out with 8 to 14 month old ob/ob mice because these Swedish obese mice are traditionally used as islet donors at this advanced age [26,29,30,31]. For comparison, we also studied islets from their lean littermates at a similar age.…”

“…Measuring [Ca 2+ ] i in whole islets is complicated by uneven loading of the fluorescent dye and the presence of several layers of cells [31,39]. However, we are confident that our results are representative of the changes occurring in the core of beta cells for two reasons: firstly, the recorded [Ca 2+ ] i changes correlate well with membrane potential changes known to occur in beta cells [21,22,23] and are similar to those observed in beta cells within islets using confocal [24,25].…”

“…The Swedish ob/ob mice are traditionally used as islet donors at an age between 8 and 13 months [26,29,30,31]. The experiments were thus carried out with 23 non-inbred female obese ob/ob mice (7.5-14 months, mean age 10.3 months) and 22 lean littermates (7.5-14 months, mean age 10.1 months) from the Umea colony.…”

Oscillatory control of insulin secretion

“…Isolated pancreatic islets not only show slow oscillations of [Ca 2+ ] i but often several-fold faster regular oscillations or slow oscillations with superimposed fast ones ( Liu et al, 1998); Table 1). Also the fast oscillations correlate with cyclic membrane depolarization with bursts of electrical activity (Santos et al, 1991).…”

“…Also the fast oscillations correlate with cyclic membrane depolarization with bursts of electrical activity (Santos et al, 1991). They somehow codepend on the Ca 2+ handling by the ER, since fast islet oscillations are immediately transformed into slow ones by SERCA inhibition (Liu et al, 1998). Like the slow [Ca 2+ ] i oscillations, the fast ones generate pulsatile insulin release from individual pancreatic islets (Bergsten, 1995).…”