Intracellular calcium concentration of acinar cells in feline tracheal submucosal glands

We measured the intracellular free calcium ion concentration ([Ca²⁺](i)) of acinar cells in isolated feline tracheal submucosal glands in response to secretagogues using the Ca²⁺-sensitive fluorescent dye fura-2. The secretagogues included cholinergic, adrenergic agonists, substance P (SP), and vasoactive intestinal polypeptide (VIP) which induce mucus glycoprotein secretion from feline tracheal submucosal glands. Methacholine (MCh) produced a significant increase in [Ca²⁺](i) of up to 9.8 times that of control in a dose-dependent fashion at concentrations of 10^-8 to 10^-3 M. [Ca²⁺](i) increase by MCh reached a peak within 30 s after stimulation and thereafter showed a sustained rise. In Ca²⁺-free medium, MCh produced an initial transient rise, which was <30% of that in a Ca²⁺-containing solution, and which lasted for 60 s with no prolonged sustained rise in [Ca²⁺](i). Atropine abolished MCh-evoked [Ca²⁺](i) increase. Phenylephrine and SP produced a prolonged increase in [Ca²⁺](i) without an initial transient increase. Phenylephrine (up to 10^-4 M) evoked an increase in [Ca²⁺](i) by up to 240% that of control, which was abolished by prazosin. SP (up to 10^-4 M) also evoked an increase in [Ca²⁺](i) by 155% that of control, which was abolished by atropine. By contrast, both isoproterenol (up to 10^-5 M) and VIP (up to 10^-5 M) failed to alter [Ca²⁺](i). These findings indicate that the mucus glycoprotein secretion evoked by muscarinic cholinergic, α-adrenergic agonist or SP can be mediated by intracellular Ca²⁺, whereas that by β-adrenergic agonists or VIP cannot.