Beta-adrenergic receptor characteristics of postnatal rat myocardial cell preparations

Primary myocardial cell cultures and freshly isolated cardiac cells in suspension represent two isolated, whole cell models for investigating cellular transsarcolemmal 45Ca++ exchange in response to a receptor-coupled stimulus. Studies were performed to characterize beta-adrenergic receptor binding, beta-adrenergic receptor mediated cellular calcium (45Ca++) exchange, and viability in purified primary myocardial cell cultures and freshly isolated cardiac cells in suspension obtained from 3- to 5-d-old Sprague-Dawley rats. In addition, beta-adrenergic receptor binding was characterized in whole-heart crude membrane preparations. All three preparations had saturable beta-adrenergic binding sites with the antagonist [125I]iodopindolol [( 125I]IPIN). The suspensions had a significantly lower Bmax (42 +/- 6 fmol/mg protein) than the membranes and cultures (77 +/- 8 and 95 +/- 10 fmol/mg protein, respectively). The KD of the cultures (218 +/- 2.0 pM) was significantly higher than that for the suspensions (107 +/- 1.3 pM) and membranes (93 +/- 1.3 pM). Viability was significantly lower in the suspensions (57%) when compared to 94% viability in myocardial cell cultures after 3 h of incubation in Kreb's Henseleit buffer. Incubation of the cultures with 5.0 X 10(-7) M isoproterenol resulted in a significant increase in 45Ca++ exchange as early as 15 s. In contrast, 45Ca++ exchange into the suspensions was not increased. Although both primary cell cultures and cardiac cells in suspension possess saturable beta-adrenergic receptors, only the monolayer cultures exhibited functional beta-adrenergic receptor-mediated 45Ca++ exchange. Of the two intact cell models investigated, these data suggest that primary myocardial cell cultures are more suitable than cell suspensions for investigating beta-adrenergic receptor binding and functions in the postnatal rat heart.