Sustained norepinephrine contraction in the rat portal vein is lost when Ca2+ is replaced with Sr2+

Agonist-induced activation of smooth muscle involves a rise in intracellular Ca2+ concentration and sensitization of myosin light chain phosphorylation to Ca2+.Sr2+ can enter through Ca2+ channels, be sequestered and released from sarcoplasmic reticulum, and replace Ca2+ in activation of myosin light chain phosphorylation. Sr2+ cannot replace Ca2+ in facilitation of agonist-activated Ca2+ dependent nonselective cation channels. It is not known whether Sr2+ can replace Ca2+ in small G protein-mediated sensitization of phosphorylation. To explore mechanisms involved in alpha-receptor-activated contractions in smooth muscle, effects of replacing Ca2+ with Sr2+ were examined in rat portal vein. Norepinephrine (NE) at >3.0 x 10(-7) M in the presence of Ca2+ resulted in a strong sustained contraction, whereas this sustained component was absent in the presence of Sr2+; only the amplitude of phasic contractions increased. Pretreatment with low (similar to0.05 mM) free Ca2+ followed by 2.5 mM Sr2+ resulted in a sustained component of the NE response. In beta-escin-permeabilized preparations, phenylephrine in the presence of GTP or guanosine 5'-O-(3-thiotriphosphate) alone induced sensitization to Sr2+. In conclusion, a Ca2+-regulated membrane/channel process is required for the sustained component of NE responses in rat portal vein. Sensitization alone is not responsible for the sustained phase of the NE contraction.