Muscarinic blockade of β‐adrenoceptor‐stimulated adenylyl cyclase: the role of stimulatory and inhibitory guanine‐nucleotide binding regulatory proteins (Gs and Gi) Journal Articles uri icon

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abstract

  • The functional antagonism that exists between muscarinic and β‐adrenoceptor function in guinea‐pig tracheal smooth muscle was investigated by assessing Gs and Gi regulated adenylyl cyclase activity in isolated membranes. Membranes from guinea‐pig tracheal smooth muscle contain both Giα and Gsα as assessed by Western blots with anti‐G‐protein antibodies. GppNHp, a non‐hydrolysable analogue of guanosine 5′‐triphosphate (GTP), was shown to stimulate adenylyl cyclase activity at high concentrations (10−6−10−4 m). GppNHp also produced a concentration‐dependent reduction in pertussis toxin‐catalysed adenosine diphosphate (ADP)‐ribosylation of Giα. Pretreatment of tracheal smooth muscle slices with methacholine (10−6 m) provoked a blockade of isoprenaline plus GTP, GppNHp‐ and GTP‐stimulated adenylyl cyclase. Addition of methacholine to membranes did not trigger inhibition of GTP‐stimulated adenylyl cyclase activity but did block the isoprenaline‐mediated augmentation of GTP‐stimulated adenylyl cyclase activity. Pretreatment of tracheal smooth muscle with methacholine (10−6 m) provoked a blockade of cholera toxin‐catalysed NAD+‐dependent ADP‐ribosylation of Gsα. Phorbol‐12‐myristate 13‐acetate (PMA)‐treatment of tracheal smooth muscle slices actually enhanced GppNHp‐stimulated adenylyl cyclase activity in subsequently prepared membranes. We suggest that methacholine in addition to inhibiting adenylyl cyclase via a Gi‐dependent mechanism induces a functional inactivation of Gs activity. These results together may explain the functional antagonism that exists between increased muscarinic tone and the ability of β‐adrenoceptor agonists to provoke excitation‐contraction uncoupling.

authors

  • Pyne, Nigel J
  • Grady, Michael W
  • Shehnaz, Darakhshanda
  • Stevens, Patricia A
  • Pyne, Susan
  • Rodger, Ian

publication date

  • November 1992

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