DEVELOPMENTAL-CHANGES IN BETA-ADRENOCEPTORS, MUSCARINIC CHOLINOCEPTORS AND CA-2+ CHANNELS IN RAT VENTRICULAR MUSCLES

1. In an attempt to explain the previous electrophysiological data on the ontogeny of β-adrenergic and muscarinic cholinergic interactions on cardiac Ca2+ current, biochemical studies were performed on the ontogeny of β-adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in cardiac muscle of developing rats: 16-20 days old foetuses, 0-20 days old neonates, and 2-3 months old adults. 2. Developmental changes in cardiac β-adrenoceptors, muscarinic cholinoceptors, and Ca2+ channels were determined with the use of specific radioligands, [3H]-dihydroalprenolol (DHA), [3H]-quinuclidinyl benzilate (QNB), and [3H]-nitrendipine (NTD), respectively. 3. The B(max) value (fmol mg-1 tissue) for [3H]-DHA binding started to increase on post-gestation day 20, reached almost its maximum level on neonatal day 6, kept almost the same level until neonatal day 20, and then decreased slightly to its adult level. 4. The B(max) value (fmol mg-1 tissue) for [3H]-QNB binding started to increase on post-gestation day 16, reached almost its maximum level on neonatal day 0, remained almost constant until neonatal day 15, and then decreased to its adult level. 5. The B(max) value (fmol mg-1 tissue) for [3H]-NTD binding increased with age between post-gestation day 18 and neonatal day 15, stayed almost constant until neonatal day 20, and then decreased to its adult level. 6. The K(d) values for [3H]-DHA, [3H]-QNB, and [3H]-NTD bindings remained almost constant during the developmental period examined. 7. Isoprenaline (Iso) increased the V̇(max) of slow action potentials (APs) from post-gestation day 18, and the adult level was reached at about 2 weeks after birth; this development time course is similar to that of Ca2+ channels. The number of β-adrenoceptors also started to increase a few days before birth, and attained its peak about one week earlier than did the V̇(max) of slow APs of the number of Ca2+ channels. 8. Acetylcholine (ACh) almost completely abolished the Iso-induced increase in V̇(max) observed from post-gestation day 18 to neonatal day 20; this developmental time course for the ACh effect is consistent with the finding that the number of muscarinic cholinoceptors started to increase on post-gestation day 16 and reached a peak on the day of birth. 9. Previous electrophysiological and the present biochemical findings strongly suggest that the functional coupling between muscarinic cholinoceptors and Ca2+ current is already established when the coupling between β-adrenoceptors and Ca2+ current starts to operate in developing rat hearts.