Figure 2. m1 Muscarinic receptors expressed in Xenopus oocytes. Arrows indicate the time when the agonist was applied. All oocytes are voltage clamped at a holding potential of 70 mV. (A) Example of a current induced by 10⁷M MCh in a defolliculated oocyte, injected with 5 ng m1 muscarinic receptor mRNA 3 days before. Charge movement is 3.7 micro C. (B) Responses to 10⁶M MCh of expressed receptors do not desensitize. The two responses are obtained from the same oocyte, separated by a wash with 10 bath volumes Tyrode's solution. Charge movements are 3.8 and 5.6 micro C, respectively. (C) Responses can be inhibited by the muscarinic antagonist atropine (1 micro Meter) and the m1-specific antagonist pirenzepine (1 micro Meter). Tracings are from four separate oocytes. Currents induced by 10⁶M MCh in the control oocytes at left are 6.6 (top) and -3.4 (bottom) micro C. If antagonist was first applied to the bath (right) only application artifact is seen.