Photolysis of caged compounds: studying Ca(2+) signaling and activation of Ca(2+)-dependent ion channels

New saliva secretion model based on the expression of Na+-K+ pump and K+ channels in the apical membrane of parotid acinar cells

The plasma membrane of parotid acinar cells is functionally divided into apical and basolateral regions. According to the current model, fluid secretion is driven by transepithelial ion gradient, which facilitates water movement by osmosis into the acinar lumen from the interstitium. The osmotic gradient is created by the apical Cl^- efflux and the subsequent paracellular Na⁺ transport. In this model, the Na⁺-K⁺ pump is located exclusively in the basolateral membrane and has essential role in salivary secretion, since the driving force for Cl^- transport via basolateral Na⁺-K⁺-2Cl^- cotransport is generated by the Na⁺-K⁺ pump. In addition, the continuous electrochemical gradient for Cl^- flow during acinar cell stimulation is maintained by the basolateral K⁺ efflux. However, using a combination of single-cell electrophysiology and Ca²⁺-imaging, we demonstrate that photolysis of Ca²⁺ close to the apical membrane of parotid acinar cells triggered significant K⁺ current, indicating that a substantial amount of K⁺ is secreted into the lumen during stimulation. Nevertheless, the K⁺ content of the primary saliva is relatively low, suggesting that K⁺ might be reabsorbed through the apical membrane. Therefore, we investigated the localization of Na⁺-K⁺ pumps in acinar cells. We show that the pumps appear evenly distributed throughout the whole plasma membrane, including the apical pole of the cell. Based on these results, a new mathematical model of salivary fluid secretion is presented, where the pump reabsorbs K⁺ from and secretes Na⁺ to the lumen, which can partially supplement the paracellular Na⁺ pathway.

Nuclear microinjection to assess how heterologously expressed proteins impact Ca2+ signals in Xenopus oocytes

The Xenopus oocyte is frequently used for heterologous expression and for studying the spatiotemporal patterning of Ca(2+) signals. Here, we outline a protocol for nuclear microinjection of the Xenopus oocyte for the purpose of studying how subsequently expressed proteins impact intracellular Ca(2+) signals evoked by inositol trisphosphate (InsP3). Injected oocytes can easily be identified by reporter technologies and the impact of heterologously expressed proteins on the generation and properties of InsP3-evoked Ca(2+) signals can be resolved using caged InsP3 and fluorescent Ca(2+) indicators.

Studying the activation of epithelial ion channels using global whole-field photolysis

The production of saliva by parotid acinar cells is stimulated by Ca(2+) activation of Cl(-) and K(+) channels located in the apical plasma membrane of these polarized cells. Here we provide a detailed description of a flash photolysis experiment designed to give a global and relatively uniform photorelease of inositol 1,4,5-trisphosphate (InsP(3)) or Ca(2+) from caged precursors (NPE-InsP(3) or NP-EGTA) combined with the simultaneous measurement of whole-cell Ca(2+)-activated currents. The photolysis light source can be either an ultraviolet (UV) flash lamp or alternatively the output from a 375-nm diode laser, which is defocused to illuminate the entire field.

Investigating ion channel distribution using a combination of spatially limited photolysis, Ca(2+) imaging, and patch clamp recording

The production of saliva by parotid acinar cells is stimulated by Ca(2+) activation of Cl(-) and K(+) channels located in the apical plasma membrane of these polarized cells. Here, we utilize a combination of spatially limited flash photolysis, Ca(2+) imaging, and electrophysiological recording to investigate the distinct distribution of Ca(2+)-dependent ion channels in the plasma membrane (PM) of enzymatically isolated murine parotid acinar cells. In these experiments, the aim of photolysis is to selectively target and modify the activity of ion channels, thereby revealing membrane-domain-specific differences in distribution. Specifically, the relative distribution of channels to either apical or basal PM can be investigated. Since there is substantial evidence that Ca(2+)-dependent Cl(-) channels are exclusively localized to the apical membrane of acinar cells, this provides an important electrophysiological verification that a particular membrane has been specifically targeted.

Analyzing Ca(2+) dynamics in intact epithelial cells using spatially limited flash photolysis

The production of saliva by parotid acinar cells is stimulated by Ca(2+) activation of Cl(-) and K(+) channels located in the apical plasma membrane of these polarized cells. Here we describe a paradigm for the focal photorelease of either Ca(2+) or an inositol 1,4,5 trisphosphate (InsP(3)) analog. The protocol is designed to be useful for investigating subcellular Ca(2+) dynamics in polarized cells with minimal experimental intervention. Parotid acinar cells are loaded with cell-permeable versions of the caged precursors (NP-EGTA-AM or Ci-InsP(3)/PM). Photolysis is accomplished using a spatially limited, focused diode laser, but the experiment can be readily modified to whole-field photolysis using a xenon flash lamp.