Role of the cytoskeleton in secretion of chloride by shark rectal gland

The salt gland of the spiny dogfish, Squalus acanthias, can be stimulated to secrete chloride by two different endogenous peptides: cardiac natriuretic peptide (CNP) and the neurotransmitter, vasoactive intestinal peptide (VIP). We examined the role of the actin cytoskeleton and of myosin light chains in this process by perfusing isolated rectal glands with and without an inhibitor of actin filament organization (cytochalasin D) and an inhibitor of myosin light chain kinase (ML-7). Cytochalasin D, 10(-6) M, reduced secretion stimulated by a 1-min bolus of CNP (5x10(-7) M) by 50-60%. In the presence of 10(-2) M procaine (which blocks neural release of VIP), cytochalasin D completely prevented CNP stimulation. In contrast, cytochalasin D did not inhibit stimulation of the rectal gland by VIP or by forskolin. Similarly, 5x10(-6)M ML-7 almost completely inhibited direct stimulation of rectal gland secretion by CNP, but did not alter chloride secretion induced by VIP or forskolin. Finally, the average time between hormonal injection and activation of secretion was 2 min longer for CNP than for VIP, consistent with the hypothesis that a contractile cellular function involving the cytoskeleton is important in CNP-induced chloride secretion, but less so when secretion is stimulated by VIP.