Ca²⁺-induced loss of Ca²⁺/calmodulin-dependent protein kinase II activity in pancreatic β-cells

Abstract

Elevations in intracellular Ca2+ in electrically permeabilized islets of Langerhans produced rapid insulin secretory responses from β-cells, but the Ca2+-induced secretion was not maintained and was irrespective of the pattern of administration of elevated Ca2+. Ca2+-insensitive β-cells responded normally to activators of protein kinase C or cAMP-dependent kinase with increased insulin secretion. The loss of secretory responsiveness to Ca2+ was paralleled by a reduction in Ca2+-induced protein phosphorylation. This was caused by a reduction in Ca2+/calmodulin-dependent protein kinase II (CaMK II) activity in the desensitized cells, as assessed by measuring the phosphorylation of a CaMK II-specific exogenous substrate, autocamtide-2. The Ca2+-induced reductions in kinase activity and protein phosphorylation were not dependent on the activation of Ca2+-dependent protein kinases and were not caused by the activation of phosphoprotein phosphatases or of Ca2+-activated proteases. The concomitant reductions in CaMK II activity and Ca2+-induced insulin secretion suggest that the activation of CaMK II is required for normal insulin secretory responses to increased intracellular Ca2+ concentrations.

Roles of phosphorylation of myosin binding protein‐C and troponin I in mouse cardiac muscle twitch dynamics

“…To elucidate the role of MyBP-C phosphorylation by CaMK2 we used the CaMK2-specific inhibitor, AIP. This inhibitor peptide has been shown to specifically inhibit CaMK2 activity in previous studies (Jones & Persaud, 1998;Vinogradova et al 2000;DeSanti-ago et al 2002). However, a decrease in the phosphorylation of MyBP-C caused a reduction in TnI phosphorylation in our study (Fig.…”

“…The experimental protocol consists of (1) increasing stimulation frequency from 0.5 Hz to 1.0 Hz, then continuing up to 8.0 Hz by 1.0 Hz increments (duration of 2 min or until a steady force value has been reached), (2) decreasing stimulation frequency to 0.5 Hz and superfusing with KH solution containing 5 μ m of myristoylated autocamtide‐2 related inhibitory peptide (AIP; Calbiochem, La Jolla, CA, USA) to inhibit CaMK2 or 1 μ m of PKA inhibitor 14–22 amide (Calbiochem) for 25 min, and (3) repeating the 0.5 Hz to 8.0 Hz increase in stimulation frequency. The specificity of the CaMK2 inhibitor, AIP, was initially shown in pancreatic β‐cells (Jones & Persaud, 1998) and has also been widely used in cardiac cells (Vinogradova et al 2000; DeSantiago et al 2002). In some experiments, the β‐adrenergic receptor blocker, propranolol (1 μ m ), was perfused for 15 min before kinase inhibitor perfusion.…”

Evidence for a sustained increase in clonal beta-cell basal intracellular Ca2+ levels after incubation in the presence of newly diagnosed Type-1 diabetic patient sera. Possible role in serum-induced inhibition of insulin secretion

“…2+ /calmodulin-dependent kinase II activity Cell extracts were prepared and Ca 2+ /calmodulindependent kinase II activity determined as previously described (Jones & Persaud 1998).…”

Effect of extracellular pH on insulin secretion and glucose metabolism in neonatal and adult rat pancreatic islets

“…Their closure results in the depolarization of the cell membrane with subsequent influx of calcium and stimulation of exocytosis of hormone granules [31][32][33]. Thus, the increase in intracellular calcium in response to glucose stimulation is well documented.…”

“…Elevations in cytosolic Ca 2+ alone are sufficient to initiate insulin secretion [34,35]. On the other side, permeabilization of islets that allows elevation of [Ca 2+ ]i independently from the metabolism results in desensitization of insulin release in response to elevation of [Ca 2+ ]i, despite retaining the ability to respond normally to other second messengers [32]. Moreover, insulin release was found to be pulsatile both at stable and oscillatory [Ca 2+ ]i, with changes in secretory rate caused by the sugar also when [Ca 2+ ]i is unchanged [36].…”