Facilitation of calcium blocking and membrane effects by intrinsic sympathomimetic activity.

The interactions between ICI 118, 587 (Corwin) a beta 1-selective partial adrenergic agonist, and atenolol (Tenormin), propranolol (Inderal) and verapamil were examined first in anaesthetised dogs pretreated with syrosingopine and vagotomised. ICI 118, 587 was administered iv in cumulative doses of 0.1 to 1000 micrograms X kg-1. In four animals, heart rate increased from 102 +/- 4 to 188 +/- 12 beats X min-1 with a KA of 2.5 +/- 0.9 micrograms X kg-1. ICI 118, 587 was then administered to five groups each of four animals pretreated with atenolol (250 micrograms X kg-1 and 500 micrograms X kg-1iv), propranolol (250 micrograms X kg-1 and 500 micrograms X kg-1) or verapamil (200 micrograms X kg-1 plus 5 micrograms X kg-1 X min-1 iv). Both atenolol and propranolol shifted the dose response curve to the right but verapamil did not. Atenolol did not lower the maximal heart rate response to ICI 118,587. Both propranolol and verapamil slowed heart rate significantly (P less than 0.05) after, but not before, ICI 118,587. Further studies were then carried out in vitro. Cat papillary muscles were superfused with physiological saline at pH 7.4 and with either atenolol or ICI 118,587 at 5 X 10(-5) mol X litre-1. Tension was recorded at Lmax over a [Ca2+] range of 0.5 to 8 X 10(-3)mol X litre-1. The pA2 for verapamil against Ca2+ in the presence of atenolol and ICI 118,587 was 5.25 +/- 0.10 and 6.57 +/- 0.22 respectively.(ABSTRACT TRUNCATED AT 250 WORDS)