Interactions between a ?calcium channel agonist?, Bay K 8644, and calcium antagonists differentiate calcium antagonist subgroups in K+-depolarized smooth muscle

Abstract

The proposal that calcium antagonists have different sites of action has been tested by attempting to reverse their inhibitory effects with a dihydropyridine which augments Ca2+ entry into cells, Bay K 8644. Bay K 8644 (1-1000 nmol/l) increased the sensitivity to Ca2+ of K+-depolarized taenia preparations from the guinea-pig caecum. Thus Bay K 8644 augmented established submaximal Ca2+-induced contractions and also shifted cumulative concentration-response curves to Ca2+ to the left, even in the presence of an optimal K+-depolarization. The inhibitory effects of nifedipine (10 nmol/l), verapamil (0.2 mumol/l), diltiazem (1 mumol/l) and diclofurime (1 mumol/l) on Ca2+-induced contractions were reversed by Bay K 8644 (1-1000 nmol/l). In contrast, Bay K 8644 did not reverse the effects of cinnarizine (1 mumol/l), flunarizine (1 mumol/l), fendiline (3 mumol/l), prenylamine (3 mumol/l), pimozide (1 mumol/l), bepridil (3 mumol/l), perhexiline (10 mumol/l) or the calmodulin antagonist W-7 (200 mumol/l). Bay K 8644 (1-100 nmol/l) was less effective at reversing the effects of nisoldipine (10 nmol/l), a slowly dissociating dihydropyridine, than the effects of nifedipine. However, preincubation with Bay K 8644 (1 mumol/l) protected the taenia from the inhibitory effects of nisoldipine (10 nmol/l). These findings are compatible with interactions of nisoldipine and Bay K 8644 at a common site. Taenia preparations incubated with Bay K 8644 (1 mumol/l) were protected from the inhibitory effects of nifedipine (10 nmol/l), nisoldipine (10 nmol/l) and to a lesser extent verapamil (0.2 mumol/l) and diltiazem (1 mumol/l).(ABSTRACT TRUNCATED AT 250 WORDS)

Direct activation of Ca2+ channels by palmitoyl carnitine, a putative endogenous ligand

“…Palmitoyl carnitine was capable of increasing Ca2"-induced contractions in the presence of a maximally-effective concentration of Bay K 8644 (Figure 2). Thus, when taenia preparations were incubated with Bay K 8644, using a concentration (1 jimol I') previously shown to be maximally effective (Spedding & Berg, 1984), and submaximally contracted with Ca2" (30 pmol I'), palmitoyl carnitine caused a further increase in the Ca2" contraction ( Figure 1); Bay K 8644 (I-lOgImollI') did not augment a Ca2"-induced contraction under these conditions.…”

“…We have therefore compared the effects of palmitoyl carnitine with those of Bay K 8644, a dihydropyridine Ca2+ channel activator (Schramm et al, 1983) in K+-depolarized smooth muscle. The K+-depolarized taenia preparation from the guinea-pig caecum has been shown to be very sensitive to Ca2+ channel activators and antagonists (Spedding, 1982;Spedding & Berg, 1984) allowing quantitive analysis of interactions between the compounds (Spedding, 1985a). Analysis of the affinity of palmitoyl carnitine for the high affinity binding sites of 3H-labelled calcium-antagonists was performed in rat cerebral cortex membranes, where affinity of agents for VOCs voltage-operated Ca2" channels) parallels inhibitory potency in smooth muscle.…”

Inhibition of calcium channels by harmaline and other harmala alkaloids in vascular and intestinal smooth muscles

“…Spedding & Berg (1984) found that the inhibitory effect of verapamil, diltiazem and dihydropyridine Ca2+ antagonists were antagonized by Bay K 8644 whereas the inhibitory effects of diphenyl alkylamine Ca2+ antagonists were not, and proposed the use of Bay K 8644 for classification of Ca2+ antagonists. Thus, the site ofaction ofharmaline on Ca2+ channels seems to be different from that of verapamil, diltiazem and dihydropyridines.…”

“…Muscle contraction reached a steady level within 30-40min and then harmaline or related alkaloids were cumulatively added at 15-20min intervals, as shown in Figure 1. In some experiments, a single concentration (2 x 10-4M) of harmaline was added during the sustained contraction to induce almost complete relaxation followed by an addition of7.5 mM Ca2+ or 10-7M Bay K 8644, and recovery of muscle contraction was observed, as described by Spedding & Berg (1984).…”

“…Ca2" channel facilitator (Schramm et al, 1983), was examined as described by Spedding & Berg (1984). In both aorta and taenia, addition of 2 x 10-4M harmaline almost completely inhibited the high K+-induced contractions and addition of 7.5mM Ca2" recovered the muscle tension.…”