Simultaneous monitoring of Ca2+ responses and salivary secretion in live animals reveals a threshold intracellular Ca2+ concentration for salivation

New Findings

What is the central question of this study?

The effects of Ca²⁺ responses on salivary fluid secretion have been studied indirectly by monitoring ion channel activities and other indices. Therefore, Ca²⁺ responses during salivary secretion remain poorly understood.

What is the main finding and its importance?

Herein, we developed a simultaneous monitoring system for Ca²⁺ responses and salivary secretion in live animals using a YC‐Nano50‐expressing submandibular gland and a fibre‐optic pressure sensor. This new approach revealed a clear time lag between the onset of Ca²⁺ responses and salivary secretion. We also estimated the [Ca²⁺]_i and provided direct evidence for the regulation of salivary secretion by small increases in [Ca²⁺]_i in submandibular gland acinar cells.

Abstract

We monitored changes in [Ca²⁺]_i during salivary secretion in the rat submandibular gland in live animals using a combination of intravital Ca²⁺ imaging with the ultrasensitive Ca²⁺ indicator YC‐Nano50 and a fibre‐optic pressure sensor. Intravenous infusion of ACh (10–720 nmol min⁻¹) increased [Ca²⁺]_i and salivary flow rate in a dose‐dependent manner. Repetitive stimulation with ACh induced equivalent Ca²⁺ responses and salivary secretion in the same individual animals. The accurate ACh stimulation experiments revealed a clear time lag between the onset of the increase in [Ca²⁺]_i and salivary secretion. The time lag with the lowest dose of ACh (30 nmol min⁻¹) was 106 s, which shortened to 19 s with the dose used for maximal salivary secretion (360 nmol min⁻¹). This time lag might reflect the time required for [Ca²⁺]_i to reach the level required to activate molecules for fluid secretion. The resting [Ca²⁺]_i in submandibular gland was 37 nm, and [Ca²⁺]_i at the onset of salivary secretion was 45–57 nm, irrespective of ACh dose. These results indicate that low [Ca²⁺]_i is sufficient to trigger fluid secretion in the rat submandibular gland in vivo.

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